Change get_all_instance_metadata to use _get_instances_by_filters

We need an instance list for invoking check_policy against when calling
get_all_instance_metadata. Since the _get_instances_by_filters
call also returns associated instance metadata, we can use this instead
of db.api.get_all_instance_metadata.

Fixes bug #1192715

Change-Id: Idb005b7227447308a8d060be7aa9ff5c70b5ff44
Merge "Change get_all_instance_metadata to use _get_instances_by_filters"