Avoid extra lookups in extra-specs-list

extra-specs-list used to use the extra-specs cinder extension
to get information on volume type extra specs. This information
is publicly available through the volume-type API [1] thus we
can avoid the extra-lookups of querying the extra-specs extension.

As the extra-spec info is publicly available, remove the part of
the docstring of cinder extra-specs-list that says 'admin only'.

[1] http://docs.openstack.org/api/openstack-block-storage/2.0/content/GET_getVolumeTypes__v2__tenant_id__types_Volume_Types.html

Change-Id: I9a8256a044511ac5233d8170fc57118ccaa1e217
Closes-Bug: 1343876
Merge "Avoid extra lookups in extra-specs-list"