Split up extracting auth.py file

The auth.py file does a couple of things, It contains keystone authentication
and to set request context. So this split up to two files.
After this commit, request hook should be included in hooks.py.

Related-Bug: #1406539
Change-Id: I1754da40383976e48f6fd4ca23911717f918f9bb
Merge "Split up extracting auth.py file"