use released library versions by default

This patch provides a new path for installing libraries in devstack so
that it's possible to either test with upstream released libraries, or
with git versions of individual libraries.

Libraries are added by name to 3 associative arrays GITREPO,
GITBRANCH, GITDIR. When we get to the library install phase we inspect
LIBS_FROM_GIT and look for libraries by name (i.e. "oslo.config") and
if they exist we'll clone and install those libraries from
git. Otherwise we won't, and just let pip pull them as dependencies
when it needs them.

This patch provides the conversion of the oslo libraries, including
pbr.

Devstack-gate jobs for these libraries will need to change to support
actually forward testing their content.

Change-Id: I6161fa3194dbe8fbc25b6ee0e2fe3cc722a1cea4
Merge "use released library versions by default"