support installing clients and supporting libraries at released versions

expand the devstack support for libraries from released versions to
support python-* clients and supporting libraries. This is a collapse
of the master patch series that took a while to get correct.

add unit tests for GIT* definitions

This adds unit tests for all the GIT* definitions, ensuring that for
libraries we think should be defined, they are. It exposed a bug in
glance_store definitions in the process.

The GITDIR definition for python-openstackclient is moved to stackrc
for testability.

Change-Id: I26fac0ccf8fd4818e24618d56bf04b32306f88f6
Merge branch 'stable/juno' of https://github.com/openstack-dev/devstack into stable/juno