Cleanup sections about Multiple Compute Nodes

common/section_multiple-compute-nodes.xml was not used everywhere.
It was introduced with change I0a7fb144241be7acf7c9063e5290220b5d7b7192
that moved content to the Config Guide, thus include it from there.

openstack-install/configuring-multiple-compute-nodes.xml is a duplicate
of common/section_multiple-compute-nodes.xml which was unused as well
and thus removed.

Change-Id: I4d4158ad033d5dd97d17f4f7f3efd0be0ca6801d
Merge "Cleanup sections about Multiple Compute Nodes"