Fix filtering aggregate metadata by key

Filtering by metadata key has no effect in the following two functions:

    aggregate_metadata_get_by_host
    aggregate_host_get_by_metadata_key

Rely on the explicit join instead of joinedload to load the _metadata
relationship in these functions (indicated by the contains_eager
option), so that filtering by key still applies when the metadata is
collected.

Fixes bug 1201283.

Change-Id: I3e44b5db8d67787bfa14f0e57d30a02a271a8833
Merge "Fix filtering aggregate metadata by key"