Make user_creds_id a parser.Stack attribute

Expose the user_creds_id column via a stack attribute, like all
the other columns, this avoids loading another stack object from
the DB just to retrieve the id, and also provides a convenient
interface to update the id when required (via Stack.store())

Partial-Bug: #1266523
Change-Id: Id404f9402b24988266acd881d1e775aa3cfee60d
Handle 503 response from Nova API

The stack fails when Nova responds with a 503 during a
create/update/delete.  503 (Service Unavailable) is an intermittent,
non-critical error and should be handled the same way OverLimit and 500
errors are being handled.

Change-Id: Iadc3b39d099fcc7870764b7a42c5edee41c527dc
Closes-Bug: #1289549
Merge "Handle 503 response from Nova API"
Merge "Make user_creds_id a parser.Stack attribute"