ntpmon should support configuration for path to chronyc binary/chrony configs

[afreiberger]

Related to the issues noted in https://bugs.launchpad.net/ntp-charm/+bug/1938721 with the co-resident installation of ntp-charm and snap installed MAAS, it would be useful to have the ability to define or detect alternate paths to the chrony client.

Path for snap installed MAAS chrony client is: /snap/maas/current/usr/bin/chronyc

[paulgear]

NTPmon doesn't hard-code paths of binaries, but accepts the system PATH setting. If a non-default chronyc is required, please set PATH appropriately in the environment, e.g. via /etc/default/ntpmon if installed with the existing PPA Debian package, or whatever EnvironmentFile is set to in your systemd service file.