Increase flexibility of stackrc repo config.

 * Offline use of devstack previously required defining individual
   repo overrides.  This change maintains support for individual
   overrides while adding the ability to override GIT_BASE.

Change-Id: I4ca8b8e69c0fb05a7c9cf2fd4643eac2f7643aa9
Add err()/err_if_not_set()

* err() and err_if_not_set() do error-like reporting without aborting the script
* die_if_not_set() now properly dies
* add is_running() from Grenade

Change-Id: I38b88112415a3c07e35bbc2dc65ad839c4d63fce
Merge "Increase flexibility of stackrc repo config."
Formatting cleanups, doc updates and whatnot

Change-Id: Ica8298353be22f947c8e8a03d8dc29ded9cb26dd
Merge "Add err()/err_if_not_set()"