Allow loading auth plugins from CLI

With a standard definition of auth plugin options we should be able to
load and use those plugins from command line applications. Provide a
mechanism to register argparse parameters and load from them.

Blueprint: standard-client-params
Change-Id: I5d9904fa885602aaaef7a9e0afd4bd6bbfca3f07
Merge "Allow loading auth plugins from CLI"
Test commit