Use the mirror type in Cluster.possibleMoves

Currently the function possibleMoves always presumes DtDrbd, which is
wrong for shared storage. To make it work with all disk templates, we
additionally pass the mirror type to it (we assume that the move
behaviour only depends on the mirror type, and not the individual
disk template; currently this is true, and we should aim to keep it
that way).

For none and external mirrors, we don't give any solutions; for
internal mirrors, we keep the previous algorithm.

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