Add instance_type extra_specs to system_metadata

With extra_specs in system_metadata, cases where db calls are needed
to fetch the extra_specs can be replaced with calls to
instance_types.extract_instance_type to get it from locally from the
instance.

Resolves bug 1134650.

Change-Id: I5b8edef84dc2b652778e364fbf886adcca0506f0