Add QA for gnt-instance modify --new-primary

Add a simple walk through the intended use case of
gnt-instance modify --new-primary for the disk template
file. The tested scenario is shutting down an instance,
manually moving the disk to the new node, setting the
new primary, and starting up the instance on the new
node. It is also verified that the operation is rejected
while the instance is still running on the old node.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>