Refactor verify signing dir logic

Add a function AuthProtocol::verify_signing_dir() in
keystoneclient/middleware/auth_token.py. It will be called both in init
stage and in fetching signing cert scenario.

Fixes bug 1201577

Change-Id: Ice0e8a93ba9d85c20de84cf8da3e9b0f2274b740
Merge "Refactor verify signing dir logic"