Allow more settings to be in /etc/clearwater/config

This allows `max_peers`, `num_http_threads`, `impu_cache_ttl` and `hss_reregistration_time` to be set in /etc/clearwater/config as well as /etc/clearwater/user_settings. user_settings takes priority, so there won't be a behaviour change unless one of those options is set in /etc/clearwater/config but not user_settings (and in that case the user probably *wants* that configuration option anyway).

`impu_cache_ttl` and `hss_reregistration_time` make much more sense in /etc/clearwater/config, because they affect Cassandra TTLs, and that's deployment-wide. `max_peers` also makes sense deployment-wide. `num_http_threads` maybe less so, but there's no harm in having the added flexibility.

Not tested - I propose to test by:
* setting hss_reregistration_time and max_peers to different values in /etc/clearwater/config and /etc/clearwater/user_settings, and checking that user_settings take priority
* setting hss_reregistration_time and max_peers in /etc/clearwater/config only, and checking that they are picked up