Add missing key attribute to AggregateList.get_by_host()

This simply adds the ability to pass the key attribute to the
by-host lookup for aggregates.

Related to blueprint compute-manager-objects

Change-Id: I7651bc1470845b1e3586aa3616809bdf2ef2b4a2
Merge "Add missing key attribute to AggregateList.get_by_host()"