Fix issue deleting ec2-credentials as non-admin user

The ec2tokens controller incorrectly uses the access id, not the
hashed credential id in _assert_owner, which means that non-admin
users can't delete their ec2-credentials.  Adding the hashing, as
in _get_credentials fixes the problem.  Test added demonstrating
the issue.

Change-Id: Ifb6e3e10a50541cf21d25880bd74e9aeb6df4f26
Closes-Bug: #1245435
(cherry picked from commit 85ca6ac8a7fab14c659673ddf47777badcbcbf04)