qa/workunits: cephtool: wait for mds active before setting map (if MDS>0)

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: MDSMonitor: don't allow mds to boot if num_up_mds() == max_mds

While running cephtool/test.sh, we set max_mds = 0 and stop the mds.
However, after a little while, the mds notices it's marked as DNE in the
mdsmap and respawns itself, leading to a boot beacon to the monitors.

The monitors would simply allow the mds to boot disregarding max_mds,
thus allowing at least 1 up mds even though max_mds was set to zero.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>