nrpe-external-master monitor script check_haproxy fails when enable_monitoring = false

When deploying haproxy and relating nrpe, check_haproxy is defined by the haproxy charm in /etc/nagios/nrpe.d/check_haproxy.cmd.

However, the command this calls in charmhelpers errors out if enable_monitoring is not set to true.

Either we need to not drop check_haproxy.cmd on nrpe-external-master relation joined/changed if enable_monitoring is set to false, or we need to override the enable_monitoring configuration if nrpe-external-master relation exists.

There are some security repercussions of automatically enabling monitoring, i.e. opening ports for stats when deployment may not intend for stats port to listen, so we should probably not have check_haproxy.cmd configured if enable_monitoring = false.

This was found on a xenial/queens cloud with 18.05 charms.