mailman monitor-only check procs by list user

only check processes run by the 'list' user,
or the checks will find themselves and we get
criticals for 9 (vs. 8) and 2 (vs. 1) procs running

thanks Alex for pointing out to use -u, that works
just as well, i usually would have put the entire
commandline starting with ^ to avoid this issue

Change-Id: Ie30bc8178e6b0ed8a282191ea85b3f5a774fc5a0
RT: 7878