Keystone Verify - Further clarity on the use of ADMIN_PASS

A user reported an issue that the documentation was not clear as to where
the "ADMIN_PASS" paramater came from. I've clarified this further and
added a link to the relevant step to enforce it's origin. Also corrected
a very minor typo.

Note that this is now aware of the way that Debian-based platforms configure
the users in Keystone and links to the relevant page for further assistance.

Change-Id: Ie1722812386591fa0563b57a4127b0042da1ee40
backport: havana
Closes-Bug: #1252281
(cherry picked from commit 6dc666cef773fafa0ff632664a7641ab75872525)
Cleaned up section_compute-configure-backing-storage

Fixed typo, removed "s"
disk should not be plural, changed from disks to disk

Change-Id: I7e0d0fdb89e26f4b3a08a4ab6251476c08220be1
(cherry picked from commit 177c61c3f2f0d4260548f1f7a5e94037ae1a02ef)
Merge "Keystone Verify - Further clarity on the use of ADMIN_PASS" into stable/havana