Add role git-prepare-nodecache

The git-prepare-nodecache is a companion role that can be used to
create a git cache suitable to be used by the prepare-workspace-git
role.

Change-Id: I623eb8abc64d956069b4f18d7a1d61383a705ac9

roles/git-prepare-nodecache/README.rst

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+Prepare an archive containing all repositories that are part of the job. This
+can be used to prepare the repos archive suitable for caching in the node
+image to be used by `prepare-workspace-git`.
+
+The path to the resulting archive file will be stored in the `git_cache_file`
+variable. That variable can be used to push the archive to a place where
+it will be picked up to be baked into the node image.
+
+**Role variables**
+
+.. zuul:rolevar:: git_cache_root
+   :default: {{ansible_user_dir }}/git-cache"
+
+   Directory where the git cache should be prepared. Usually this should not
+   be changed.

roles/git-prepare-nodecache/defaults/main.yaml

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+git_cache_root: "{{ansible_user_dir }}/git-cache"

roles/git-prepare-nodecache/tasks/main.yaml

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+- name: Create git parent dirs
+  file:
+    path: "{{ git_cache_root }}/{{ item.canonical_name | dirname }}"
+    state: directory
+  with_items: "{{ zuul.projects.values() | list }}"
+
+- name: Copy git repos to {{ git_cache_root }}
+  command: cp -r "{{ ansible_user_dir }}/{{ item.src_dir }}/.git"
+    "{{ git_cache_root }}/{{ item.canonical_name }}"
+  with_items: "{{ zuul.projects.values() | list }}"
+
+- name: Mark repos as bare
+  command: git config --bool core.bare true
+  args:
+    chdir: "{{ git_cache_root }}/{{ item.canonical_name }}"
+  with_items: "{{ zuul.projects.values() | list }}"
+  # We don't want git module foo, just set a config value.
+  tags:
+    - skip_ansible_lint
+
+- name: Run git garbage collection
+  command: git gc
+  args:
+    chdir: "{{ git_cache_root }}/{{ item.canonical_name }}"
+  with_items: "{{ zuul.projects.values() | list }}"
+  # The ansible git module doesn't support garbage collection.
+  tags:
+    - skip_ansible_lint
+
+- name: Set git_cache_file variable
+  set_fact:
+    git_cache_file: "{{ git_cache_root }}/repos.tar.gz"
+
+- name: Create repos tarball
+  shell: |
+    tar -czf "{{ git_cache_file }}" *
+  args:
+    chdir: "{{ git_cache_root }}"
+  # Ansible lint wants we should use the unarchive module but we want the other
+  # way round.
+  tags:
+    - skip_ansible_lint