Run instance hooks on more nodes

This should fix issue 68: some hooks should be run on more nodes than
currently. GrowDisk runs on both nodes, remove run the post hook on the
instance's nodes, and failover and migrate run the post hook on the
source node too.

Thanks to Maxence for the initial investigation and patch.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>