add get_by_metadata_key to AggregateList object

This change adds the get_by_metadata_key function to the AggregateList object
and the aggregate_get_by_metadata_key function to the DB API.

Previously, the only DB API function available for finding aggregates matching
a metadata key returned a dictionary of aggregated metadata. The
aggregate_get_by_metadata_key function returns all rows matching the specified
metadata key, and is called by the existing aggregate_host_get_by_metadata_key
DB API function.

The get_by_metadata_key function returns a list of Aggregate objects matching
the specified metadata key.

Related to blueprint compute-manager-objects-juno

Change-Id: If50c9f613dd192ab44577f26a81dd5b40e3a7af7
Merge "add get_by_metadata_key to AggregateList object"