Use the new dry-run mode in cmdlib

This will hopefully detect potential LVM (or any other storage, when
they implement it) issues before committing changes just on some
nodes.

Unfortunately due to the dry_run opcode handling, we can't integrate
this into the usual handling (as we need to activate the disks before
doing any tests, which belongs in Exec not in CheckPrereq).

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