Don't require an admin user if there isn't a need.

This change makes it possible to skip specifying if a user has to be
an admin or not as in some contexts it may not matter. Found while
trying to run the tests against an environment where my Nova user
worked only for the Nova MGMT api.

implements blueprint tests-should-not-require-admin-prop
Change-Id: Ief62d47216b6366e36fd90e86baf81172e5281ba
Merge "Don't require an admin user if there isn't a need."