Cherry-pick: Fall back to pkill in run_ctrl_c.sh

The pkill executable is provided by the same package that provides ps
which is why it is more likely to be installed. Seems that the test VMs
don't always have killall installed.

Run the MaxScale process with the maxscale user instead of the root user
to prevent any new files from being owned by a different user.

Cherry-picked the commit a06c122a5e050071589f724758d6bbd3d8d3f419 from
develop.

maxscale-system-test/test_ctrl_c/start_killer.sh

@@ -1,3 +1,3 @@
 sleep 5
-sudo /usr/bin/killall maxscale -s INT
+sudo /usr/bin/killall maxscale -s INT || sudo pkill -INT maxscale
 

maxscale-system-test/test_ctrl_c/test_ctrl_c.sh

@@ -10,7 +10,7 @@ fi
 
 T="$(date +%s)"
 
-sudo maxscale -d -U root
+sudo maxscale -d -U maxscale
 if [ $? -ne 0 ] ; then
 	exit 1
 fi