[chrony] Add ability to disable clients using as an NTP server

[Haw Loeung]

Hi,

The charm ships a chrony config as follows[1]:

| [hloeung@ackpow etc]$ cat /etc/chrony/chrony.conf | grep '^allow' -B2
| # allow any host to use us as an NTP server
| # (use juju unexpose to prevent access)
| allow

On edge hosts not deployed in OpenStack or public cloud providers, this allows any host to use "us" as an NTP server.

Can we please have the ability to disable this "feature" in the charm?

Thanks,

Haw

[1]https://git.launchpad.net/ntp-charm/tree/templates/chrony.conf#n40