Fixes for Tuskar API Keystone authentication

Rather than duplicating the instructions as suggested in the bug,
I added a link to the Keystone service configuration page instead.
It offers a much more comprehensive explanation of the steps than
we should.

In verifying those instructions, I had to fix code in Tuskar to
get the authentication working. The original code was copied from
Ironic, however, their needs are slightly different as they have
the notion of public APIs. They use the AdminAuthHook to facilitate
admin and public access. Furthermore, the code Tuskar did have
would crash when attempting to call to_dict() on the
context, which is specifically implemented in Ironic.

Change-Id: I13c249ea02c5c74fb378faf41ada0efb55ddf162
Closes-Bug: #1284133
(cherry picked from commit 5a9b3f19b4aace1471b178b18b96a00dfc95529b)
Merge "Fixes for Tuskar API Keystone authentication" into stable/icehouse