centos 7 mirror: switch upstream provider

kernel.org has been rejecting rsync attempts with an over-capacity
message for several days now.  Switch to the facebook mirror which
seems to be working for 8-stream.

Change-Id: I98de9dd827a3c78a023b677da854089593d5a454

playbooks/roles/mirror-update/files/centos-mirror-update

@@ -69,7 +69,7 @@ $K5START ${RSYNC} \
 # -- Centos 7 --
 #
 
-MIRROR="rsync://mirrors.kernel.org/centos"
+MIRROR="rsync://mirror.facebook.net/centos/"
 REPOS="7"
 
 for REPO in $REPOS; do