Implement cluster rename operation

This patch adds a new OpCode (and corresponding LU) that implements the
cluster rename functionality.

This is done by shutting down the master role, making the needed sstore
modifications and distributing the changed files to all nodes, and then
re-enabling the master role.

The modification to the man page of gnt-cluster also moves the section
on gnt-cluster destroy in order to correct alphabetical ordering.

Reviewed-by: imsnah