Update compute flavor module for 2.0.0

Switch sdk calls to use the proxy layer where sensible.

Ensure that returned resource objects are converted to dicts.

Removes undocumented id return value.

Rename flavorid to id. Keep flavorid as an alias for backward
compatibility.

Rename the test role from nova_flavor to compute_flavor to keep naming
consistent.

Fold tests from compute_flavor_info into the compute_flavor role.

Add additional tests to improve coverage.

Update return docs

Change-Id: I5419d1c02b9b50625beb3bff88c8e4a4f1c14667