Re-write comment for ADMIN_TOKEN

It seems that there is a misconception around the ADMIN_TOKEN in
keystone. Currently the comment read that it is a "shared secret"
and can be used between openstack services.

However this token has no user associated with it, and no
authorization. It's a method to bootstrap into keystone, that's
all.

Closes-Bug: 1261622
Co-Authored-By: Dolph Mathews <dolph.mathews@gmail.com>

Change-Id: I6c913a8079bb84b56ac69249e7d25bc94cf32dd0
Merge "Re-write comment for ADMIN_TOKEN"