Avoid pollution and false positives with tox.ini

- adapted testenv build to build in a tmp folder outside of src
- removing sitepackages in tox.ini to avoid test env pollution
- skip_missing_interpreters in tox.ini set to False to avoid false
positives by skipping missing interpreters.

Change-Id: I4d7814a557bb17afb932ddcff6c75f2a8f61f51e

tox.ini

@@ -1,24 +1,36 @@
 [tox]
 skipsdist = True
 envlist = pep8
+toxworkdir = /tmp/tox
+# NOTE(beisner): Avoid build/test env pollution by not enabling sitepackages.
+sitepackages = False
+# NOTE(beisner): Avoid false positives by not skipping missing interpreters.
+skip_missing_interpreters = False
 
 [testenv]
-basepython = python2.7
+basepython = python3
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
          TERM=linux
-         JUJU_REPOSITORY={toxinidir}/build
+         JUJU_REPOSITORY={envdir}/tmp/build
 passenv = http_proxy https_proxy
 install_command =
   pip install {opts} {packages}
 deps =
   -r{toxinidir}/requirements.txt
-whitelist_externals = /bin/true /bin/echo
+whitelist_externals = /bin/true /bin/echo /bin/mkdir /bin/ln
 
 [testenv:build]
+# ``charm build`` refuses to output to a subdirectory to the source tree
+# The gate check will look for and validate the built artifacts in the source
+# tree.
+# Build the artifats under /tmp and link back to source directory to alleviate.
 commands =
   /bin/echo 'WARNING: *build* target is for testing only.'
-  charm-build --log-level DEBUG -o {toxinidir}/build .
+  /bin/mkdir -p {envdir}/tmp
+  charm-build --log-level DEBUG -o {envdir}/tmp/build .
+  /bin/ln -s {envdir}/tmp/build/builds {envdir}/tmp/build/trusty
+  /bin/ln -s {envdir}/tmp/build {toxinidir}/build
 
 [testenv:venv]
 basepython = python3
@@ -48,6 +60,12 @@ deps = -r{toxinidir}/test-requirements.txt
 # TODO: Need to write unit tests then remove the following command.
 commands = /bin/true
 
+[testenv:py37]
+basepython = python3.7
+deps = -r{toxinidir}/test-requirements.txt
+# TODO: Need to write unit tests then remove the following command.
+commands = /bin/true
+
 [testenv:pep8]
 basepython = python3
 commands = /bin/true