Add check for DNS resolution

[Drew Freiberger]

We should add a check for DNS resolution working in good fashion.

Checking both for timeouts and ability to look up a particular address.

Defaults should probably be warn at 10 seconds, crit at 30 seconds, lookup archive.ubuntu.com.

This will help us identify upstream and cloud DNS issues.

[Drew Freiberger (afreiberger)]

This could be very noisy on a cloud if enabled by default. Suggest we might want to make it a disabled option by default, but allow for it to be enabled as needed on smaller deploys of the charm, such as nrpe-lxd-service-checks that could get related to something like landscape, nagios, graylog. Or perhaps this could be builtin to the nagios charm itself.

[Drew Freiberger (afreiberger)]

This is already part of openstack-service-checks. May be invalid for these charms, but should be considered for future nagios charm, as openstack-service-checks should be upstreamed into each individual service's checks eventually.