doc: clarify that mons must have qurorum during deploy

If multiple mons are specified in 'ceph-deploy new', but less than
those necessary for quorum are started with 'ceph-deploy mon create',
later steps (like 'ceph-deploy gatherkeys') will fail.
Merge pull request #759 from jcfischer/master

doc: clarify that mons must have qurorum during deploy