Add aggregates scenario test

The basic scenario for aggregates consists in the follow steps:
- Creates an aggregate within an availability zone
- Adds a host to the aggregate
- Checks aggregate details
- Updates aggregate's name
- Removes host from aggregate
- Deletes aggregate

cherry-picked from I7a11b11468a5bbe85da4f6154308f7a5336c6919 with one change
in the admin credentials provider method (credentials() method).

Related to bug: #1023131

Change-Id: I7bb651497dfa3ce43ed2c5ac483360829ad4ee3f
Merge "Add aggregates scenario test" into stable/havana