Expose Extra Flavor Data

[carolynvs]

http://developer.openstack.org/api-ref-compute-v2.1.html#flavor-extra-specs-v2.1

Vanilla OpenStack doesn't have this extension installed by default but Rackspace has it (not the os-extra-specs endpoint, but the data is returned by default when listing flavors).

Ported from openstacknetsdk/openstack.net#260.