Document authentication plugins

Explain the plugins that are provided and some information on how to
write new ones. This will obviously need to be expanded as more features
are added.

Change-Id: Ie67e93f2ed994c3702ac66e388cf216609dd44ff
Add v2 Token manager authenticate tests

This function is currently completely untested. Create tests to
reinforce behaviour.

Change-Id: I8cfbea37964e3d96cc444ad6f62542786429e44b
Test that tenant list function can use auth_url

Listing tenants can be done on the auth URL because it is often needed
to be performed with an unscoped token. The behaviour is not tested
though so add a test to cover it.

Change-Id: Ic14d54f0f1d8cca2c3a5a6a6da7465c703773aa0
remove useless part of error message

also did a bit of code cleanup for consistency

Change-Id: I48832c36e1a8b5a0029598dc74aaeaa3b3d2a66f
add deprecation warning for auth_token

Change-Id: I1faaae0373a7ae12ee528ab5c71741a2b1122d43
Closes-Bug: 1342274
Fix mistakes in token fixtures

There are two small mistakes i discovered in the token fixtures. Fix
these and add a bunch of new tests that would have caught them earlier.

Closes-Bug: #1342513
Closes-Bug: #1342515
Change-Id: I7c07bd915fd84cc77cae0004220d8bb8d79bb082
Merge "Document authentication plugins"
Merge "add deprecation warning for auth_token"
Merge "remove useless part of error message"
Merge "Fix mistakes in token fixtures"
Merge "Add v2 Token manager authenticate tests"
Merge "Test that tenant list function can use auth_url"