qa: fix TestInstanceModifyPrimaryAndBack flakiness

The test tries to scp the instance disks from the primary node to the
new destination, but connects to the master node instead. Thus it works
only when the primary node is the master node. Fixing by correcting to
the right node.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>