User and Database List in Create Not Validated

Fix #1: Every database in instance.users.databases should exist in
instance.databases, otherwise the guest will attempt to grant
privileges to a database that can't possibly exist.

Fix #2: The (name, host) of each user in instance.users should be
unique, otherwise there is a semantic duplicate.

Note: instance.users.databases is already checked for uniqueness via
apischema's uniqueItems assertion.

Change-Id: Ib0a148a1d7dcf6adf2a899b15db815273e5688a4
Closes-Bug: #1219627
Merge "User and Database List in Create Not Validated"