Fix a few tests that hard-coded the number of nodes

Instead of randomly generating the number of required nodes or
hard-coding it, compute it from the disk template of the
instance. Otherwise tests will fail when the disk template is not
DRBD8 and we ask for allocation on 2 nodes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>