Move TLS cert to a better location

[dstathis]

Bug Description

We need to move the TLS cert and key to a better location than /tmp. Somewhere such as /etc/grafana-agent/. This needs to be done in both charms, however, for the machine charm we can only do this once the snap is modified to allow reading from a different location.

To Reproduce

Environment

Relevant log output

-

Additional context

No response

[lucabello]

We should be able to already put it in the grafana-agent snap folder; that folder should also be accessible by the charm.

[dstathis]

We can put the certs in a different place for machine and k8s. For the machine charm we should use $SNAP_DATA similarly to what we did for positions_dir.