Support Extensions to Trove Client

extensions on the contrib path (i.e. troveclient/v1/contrib/*.py),
extensions on the python path that follow the naming convention of
*_python_troveclient_ext, and entry-points will now be discovered
and loaded.

due to the mismatch of the api version and the troveclient
package name, '1.0' vs. '1', the client has been patched to look
for contrib in troveclient/v1 vs. the standard troveclient/v1_0

Change-Id: I3ea7bb5ba471b0d421581dcfda7216f429cb7b65
Closes-Bug: #1267577
Merge "Support Extensions to Trove Client"