[rackspace|compute_v2] Flavor list returns details

This avoids forcing iteration through the flavor list and calling get on
each member to obtain the full details of flavors.
[rackspace|compute_v2] Image list returns details

Ensure consistency with detailed Flavor lists
[rackspace|compute_v2] tests for new requests

Add tests to cover the _detail variants of requests for flavor and image
information.
Merge pull request #2093 from cwuest/rackspace_compute_v2_flavors_detail

[rackspace|compute_v2] Flavor list returns details