Moved content shared with config guide to common directory.

Identified shared by the openstack-config guide and one or more other
guides and moved to the 'common' directory. Impacted href and fileref
values were also updated.

Guides involved were:

* openstack-compute-admin
* openstack-install
* openstack-config

Change-Id: I5074a65e925ebdaed7a8dff95619287c97be7562
Merge "Moved content shared with config guide to common directory."