be a bit more explicit about 'ceph-deploy new' in quickstart

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
Merge pull request #2206 from ceph/wip-8875

doc: be a bit more explicit about 'ceph-deploy new' in quickstart

Reviewed-by: John Wilkins <john.wilkins@inktank.com>