Details the proper way to call a callable

Change-Id: I288822ed3aa8a755fc90fbf7940da043995cb719
Fix invalid self link in get access token

When getting a single access token, the `self` link didn't contain user section.
This commit fixes the url by overriding the method that generates the self
referential links and adds corresponding test.

Change-Id: I366ee2d691008b6b7412adb8cf95179129310728
Closes-Bug: #1279496
Merge "Details the proper way to call a callable"
Merge "Fix invalid self link in get access token"