documentation: add centos 7 to ceph-deploy installation

http://tracker.ceph.com/issues/10246 Fixes: #10246

Signed-off-by: Loic Dachary <ldachary@redhat.com>
Merge pull request #3081 from dachary/wip-10246-centos7

doc: add centos 7 to ceph-deploy installation

Reviewed by: John Wilkins <jowilkin@redhat.com>