This is an initial pass at plugin infrastructure for devstack which
allows specifying an external repository via:
enable_plugin <name> <giturl> [branch]
It implements the devstack specification for this at
I173dee3d57967b1d2ffd30e4868a2832aeac97ce
Change-Id: I8e4175313b3cf0b12e981122358b1288a7eb0746