doc: add CentOS 7 to recommended OSes

CentOS 7 was missing from the list of recommended OSes for Firefly. Add
it to the list.

Reported-By: Frank Even <lists+ceph.com@elitists.org>
Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
Merge pull request #3060 from ktdreyer/doc-os-recommendations-centos-7

doc: add CentOS 7 to recommended OSes

Reviewed-by: John Wilkins <jowilkin@redhat.com>