Replace get_instance_metadata call in api.ec2.cloud._format_instances

The _format_instances method in api.ec2.cloud populates an instance
dictionary from the sqlalchemy instance model objects, and populates the
metadata by calling compute.api.get_instance_metadata.

This is unnecessary, since the instance objects already have metadata
attached. The helper class is needed until the migration to nova objects
is complete.

Partially fixes bug #1192715

Change-Id: Ib5a593bd36c8d6a531b9494b9861cca9a40ed08a
Merge "Replace get_instance_metadata call in api.ec2.cloud._format_instances"