ceph: Use pseudo-random numbers to choose mds

We don't need to use up entropy to choose an mds,
so use prandom_u32() to get a pseudo-random number.

Also, we don't need to choose a random mds if only
one mds is available, so add special casing for the
common case.

Fixes http://tracker.ceph.com/issues/3579

Signed-off-by: Sam Lang <sam.lang@inktank.com>