Cache zanata cli tools

Cache zanata cli tools during build, and up the prep-zanata role to
look for the cached version in preference to dowloading.

Change-Id: If70f3386d0350c94dd1548e7f3c60a1d02914824

nodepool/elements/cache-devstack/README.rst

@@ -1 +1,17 @@
-Pre-cache all of the things devstack might need
+cache-devstack
+==============
+
+Pre-cache a range of things into CI images.  This element uses the
+``source-repositories`` element to acquire files to be cached.  The
+standard cache location is ``/opt/cache/files``.
+
+A number of strategies are used to get the files to be cached.
+
+We have a number of ``source-repository-*`` files for each package
+package that should be cached into images.
+
+``extra-data.d/55-cache-devstack-repos`` goes through each devstack
+branch and runs the ``tools/image_list.sh`` script to dynamically
+build a list of files to cache as requested by devstack.  This is
+mostly virtual machine images, but also some other peripheral packages.
+

nodepool/elements/cache-devstack/source-repository-zanatacli

@@ -0,0 +1 @@
+zanata-cli file /opt/cache/files/zanata-cli-3.8.1-dist.tar.gz https://search.maven.org/remotecontent?filepath=org/zanata/zanata-cli/3.8.1/zanata-cli-3.8.1-dist.tar.gz

roles/prep-zanata/README.rst

@@ -27,3 +27,8 @@ Prepare zanata client use
    :default: 3.8.1
 
   The version of zanata client to install
+
+.. zuul:rolevar:: zanata_client_checksum
+   :default: cc4ecfa2d530ff314076bd0173bdf829824737d48d4f4a2f4ca18d263e9da7c3
+
+  The expected SHA256 checksum of the zanata client

roles/prep-zanata/defaults/main.yaml

@@ -1,2 +1,3 @@
 ---
 zanata_client_version: 3.8.1
+zanata_client_checksum: cc4ecfa2d530ff314076bd0173bdf829824737d48d4f4a2f4ca18d263e9da7c3

roles/prep-zanata/tasks/main.yaml

@@ -14,12 +14,30 @@
     state: directory
   become: true
 
-- name: extract zanata client tarball
+- name: Look for cached zanata client
+  stat:
+    path: "/opt/cache/files/zanata-cli-{{ zanata_client_version }}-dist.tar.gz"
+    checksum_algorithm: sha256
+  register: cached_client
+
+- name: Ensure correct checksum of cached client
+  assert: cached_client.checksum == "{{ zanata_client_checksum }}"
+  when: cached_client.exists
+
+- name: Extract cached client tarball
+  unarchive:
+    src: "/opt/cache/files/zanata-cli-{{ zanata_client_version }}-dist.tar.gz"
+    dest: "/opt/zanata"
+    creates: "/opt/zanata/zanata-cli-{{ zanata_client_version }}/bin/zanata-cli"
+  when: cached_client.exists
+
+- name: Download and extract zanata client tarball
   unarchive:
     src: "https://search.maven.org/remotecontent?filepath=org/zanata/zanata-cli/{{ zanata_client_version }}/zanata-cli-{{ zanata_client_version }}-dist.tar.gz"
     remote_src: yes
     dest: "/opt/zanata/"
     creates: "/opt/zanata/zanata-cli-{{ zanata_client_version }}/bin/zanata-cli"
+  when: not cached_client.exists
 
 - name: ensure zanata-cli perms
   file: