doc: Clean up pre-flight documentation
Mention recent Ceph releases.
Move important message about sudo and ceph-deploy closer to the use of
ceph-deploy.
Mention files created by ceph-deploy comment
Separate apt-get from yum command

Signed-off-by: Kevin Dalley <kevin@kelphead.org>
Merge pull request #1829 from nereocystis/preflight-cleanup

doc: Clean up pre-flight documentation