doc: quick-ceph-deploy cleanup

Improve documentation in quick-ceph-deploy.rst
Use admin-node consistently.
ceph should be installed on admin-node for the following reasons:
 "ceph-deploy admin admin-node" assumes that /etc/ceph exists.
 "ceph health" requires the use of ceph

Signed-off-by: Kevin Dalley <kevin@kelphead.org>