add scripts for performing YAML validation

this commit adds scripts for validating YAML documents, as well as a
wrapper for git pre-commit hooks.  The ci-scripts/validate-yaml
script will check individual files or entire directories, e.g:

    ./ci-scripts/validate-yaml playbooks

The ci-scripts/pre-commit-hook script is meant to be installed as a
git pre-commit hook:

    ln -s $PWD/ci-scripts/pre-commit-hook .git/hooks/pre-commit

This will run any executable scripts in `ci-scripts/pre-commit.d`.
There is one scripts there, `ci-scripts/pre-commit.d/validate-yaml-in-commit`, which will validate all of the .yml documents contained in a commit.  This can prevent CI failures do to simple syntax errors by ensuring that the yaml documents are correct before submitting a change to gerrit.

Change-Id: I088e7b56a066e9b8520af9d54489b8a61d8bc29b

ci-scripts/pre-commit-hook

@@ -0,0 +1,40 @@
+#!/bin/sh
+
+# Install this into .git/hooks/pre-commit
+
+echo "running commit checks"
+
+(
+tmpfile=$(mktemp -t gitXXXXXX)
+trap "rm -f $tmpfile" EXIT
+
+for x in ci-scripts/pre-commit.d/*; do
+    [ -x "$x" ] || continue
+
+    tput setaf 3
+    printf "%-50s" $x
+    tput sgr0
+
+    if $x > $tmpfile 2>&1 ; then
+        tput setaf 2
+        printf "[OK]\n"
+        tput sgr0
+    else
+        tput setaf 1
+        printf "[OK]\n"
+        tput sgr0
+        cat $tmpfile >&2
+        exit 1
+    fi
+done
+)
+
+if [ $? -ne 0 ]; then
+    tput setaf 1
+    echo "*** COMMIT CHECK FAILED ***" >&2
+    tput sgr0
+    exit 1
+fi
+
+exit 0
+

ci-scripts/pre-commit.d/validate-yaml-in-commit

@@ -0,0 +1,18 @@
+#!/bin/sh
+
+SCRIPTS="$PWD/ci-scripts"
+
+tmpdir=$(mktemp -d commitXXXXXX)
+trap "rm -rf $tmpdir" EXIT
+
+# This checks out the index into a temporary directory, which is
+# necessary if you want to validate the files that are actually
+# part of the commit (because when performing an interactive add,
+# for example, files in a commit may not actually match the content
+# of files in the working directory).
+
+git checkout-index --prefix=$tmpdir/ -af
+git diff --cached --name-only --diff-filter=ACM | grep '.yml' | (
+cd $tmpdir
+xargs --no-run-if-empty $SCRIPTS/validate-yaml
+)

ci-scripts/validate-yaml

@@ -0,0 +1,94 @@
+#!/usr/bin/python
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+import argparse
+import logging
+import sys
+import yaml
+
+
+def parse_args():
+    p = argparse.ArgumentParser()
+    p.add_argument('--verbose', '-v',
+                   action='store_const',
+                   const='INFO',
+                   dest='loglevel')
+    p.add_argument('--fail-fast', '-x',
+                   action='store_true')
+    p.add_argument('path', nargs='+')
+    p.set_defaults(loglevel='WARNING')
+    return p.parse_args()
+
+
+def validate_file(path):
+    global args
+    valid = True
+
+    logging.info('checking: %s', path)
+    with open(path) as fd:
+        try:
+            yaml.safe_load(fd)
+        except yaml.error.YAMLError as error:
+            valid = False
+            logging.error('%s failed validation: %s',
+                          path, error)
+
+    return valid
+
+
+def validate_directory(path):
+    global args
+    all_valid = True
+
+    for dirpath, dirnames, filenames in os.walk(path):
+        for filename in filenames:
+            if not (filename.endswith('.yml')
+                    or filename.endswith('.yaml')):
+                continue
+
+            filename = os.path.join(dirpath, filename)
+            all_valid = validate_file(filename) and all_valid
+            if not all_valid and args.fail_fast:
+                break
+        else:
+            continue
+
+        break
+
+    return all_valid
+
+
+def main():
+    global args
+
+    args = parse_args()
+    logging.basicConfig(level=args.loglevel)
+
+    for path in args.path:
+        if os.path.isdir(path):
+            all_valid = validate_directory(path)
+        elif os.path.isfile(path):
+            all_valid = validate_file(path)
+        else:
+            logging.error('%s is not a file or directory (skipping)',
+                          path)
+
+        if not all_valid and args.fail_fast:
+            break
+
+    return 0 if all_valid else 1
+
+if __name__ == '__main__':
+    sys.exit(main())