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>
Merge remote-tracking branches 'athanatos/wip-10059', 'athanatos/wip-10150', 'athanatos/wip-10157', 'athanatos/wip-8011', 'tchaikov/wip-fix-safe_read_file', 'upstream/master', 'upstream/wip-10080' and 'upstream/wip-crush-straw' into wip-sam-testing