Add solum builder client

This client will be used to communicate with builder service from solum.

Generalize KeystoneAuthPlugin so that it can be used by both solumclient and
builder client.

Fixed a bug in solumclient/common/base.py

Change-Id: I86630f40b32c6fcada5aa292fae8e36cecab9908
Merge "Add solum builder client"