Allow external calling context to inject values.

To allow non-core projects to use devstack-gate to test their changes without
needing to modify devstack-gate itself, allow injection of values into
PROJECTS and ENABLED_SERVICES from the calling script. Also let the calling
script drop in a localrc fragment and tell devstack gate to not delete it.
That way a jenkins job could set those values and then call an unmodified
devstack-gate.

Change-Id: I12802e3fedd8553e8401dcfaeaa62f8750609b69
Merge "Allow external calling context to inject values."