Recovery information should rebuild not occur

Adding steps to the troubleshooting guide to enable a user to stand
a controller node back up into a partially functional state such that
pre-flight check will pass and the upgrade be executed.

This is important if the rebuild of an overcloud node fails to occur
due to a failure on the undercloud, the state drive mounted at /mnt
will no longer be mounted and must be re-attached before proceeding.

Change-Id: I64bf7e31903dbf188d4be29e2f08bc26fb73129c