prometheus.yml should be saved to the common dir

[Simon Déziel]

Currently, the prometheus snap expects to find its configuration in `/var/snap/prometheus/current/prometheus.yml`

Being in the `current` dir means it will be lost on snap refreshes. Furthermore, any relative path used in that config will also be relative to the `current` directory. When using TLS certs as recommended in LXD doc (https://documentation.ubuntu.com/lxd/en/stable-5.21/metrics/#configure-prometheus-to-scrape-from-lxd) we'd either need to use fully qualified path to a `common` directory or risk putting the TLS files in the `current` directory that might eventually go away.

Please consider using `common` for long term file storage.