Add option to enable remote monitoring

[jdevera]

Setting the ALLOW_REMOTE_MON to true in the container will enable the server to receive chronyc commands from other hosts, using the -h $host option.

This can be useful in docker compose scenarios where a second container can be used to provide the outside world with statistics about the running NTP server without this second container needing to run docker commands.

In my case, I am launching two containers from a docker compose, and in the second container I am exposing an HTTP API that will in turn call chronyc commands (with -h ntp and return their results. I thought it could be a useful contribution.