move mailman queue monitor script to /usr/local

custom plugins should be in /usr/local, not mixing
with the directory used by the nagios-plugins package

also, the name was still wrong, did not start with "check_"
as defined in the nrpe command

Change-Id: I17eef867aa307c4a2925a37f1a361ea1455bc908
Merge "move mailman queue monitor script to /usr/local" into production