Add the galera_client dependency to the repo build

This change is required because without it the client libraries
used to build the MySQL-Python packages will depend on the base
system and they will be wrong.

In the original backport, galera_client was set as a dependency
of the repo_build role. However, due to galera_client's
dependency on pip_lock_down in liberty, the galera_client role
must be invoked with pip_no_index=false.

This is an example of the issue:
  http://paste.openstack.org/show/543424/

This is the same command when this fix is in place:
  http://paste.openstack.org/show/543436/

Backport for change I6536993523f02ba49da741b9ea21535eb33fe402

Change-Id: I8e62bf2e09217e5fb47d021dfa7f754c09297c62
Closes-Bug: #1565128

playbooks/repo-build.yml

@@ -36,13 +36,16 @@
         - repo-build-venvs
         - repo-create-venv-checksum
   roles:
+    - role: "galera_client"
     - role: "repo_build"
       repo_build_release_tag: "{{ openstack_release }}"
       repo_build_pip_default_index: "https://rpc-repo.rackspace.com/pools"
       repo_build_pip_extra_index: "https://pypi.python.org/simple"
       tags:
         - "repo-build"
   vars:
+    pip_no_index: false
+    pip_install_options: "--isolated"
     pkg_locations:
       - ../
       - /etc/ansible/roles