Make metadata server use Instance.get_flavor()

This removes the use of extract_flavor() from the metadata server.

Related to blueprint flavor-from-sysmeta-to-blob

Change-Id: I06373f02a693d5fa9ad5acfe4e268a5fdbd5bac8
Make network/* use Instance.get_flavor()

This makes the nova-network and neutron API modules use the
get_flavor() helper to avoid dependence on flavors being in
system_metadata.

Related to blueprint flavors-from-sysmeta-to-blob

Change-Id: I11033a755e577031773bf01634f020455844e625
Merge "Make metadata server use Instance.get_flavor()"
Merge "Make network/* use Instance.get_flavor()"