TT#36053 Make systemd sems service require redis-master

This service requires write access to redis, and relies on redis-master
to setup Redis in that mode. If we switch the redis master/slave roles
while SEMS is still running, or while we are trying to shut it down,
it gets very confused and takes very long to realize and shutdown.
So we should always make sure that redis-master is running before we
start sems, and stop sems before we stop redis-master.

Change-Id: Iaa43186230df6ab8365c94585192fbb6db2652cc

debian/ngcp-sems.service

@@ -1,9 +1,11 @@
 [Unit]
 Description=NGCP SIP Express Media Server (SEMS)
 After=mariadb.service
+After=redis-master.service
 After=network-online.target
 After=remote-fs.target
 Requires=mariadb.service
+Requires=redis-master.service
 Requires=remote-fs.target
 Wants=network-online.target