Move config parsing and validation into config.go

[pudelkoM]

This change collects all the checks, parsers and validations littered around, and puts them in one place.
Defaults are now set in one place and at startup, so the full config can be logged and inspected.

[osinstom]

It looks like JSON files for integration tests must be updated.

[osinstom]

just one concern on moving ue_ip_pool and enable_ue_ip_alloc to the root level of the JSON file: are we worried about breaking existing deployments? We would need to update a lot of deployment files..

CC: @ccascone

[pudelkoM]

just one concern on moving ue_ip_pool and enable_ue_ip_alloc to the root level of the JSON file: are we worried about breaking existing deployments? We would need to update a lot of deployment files..

Good point, I think we can skip on the move part. But ip pool should still be removed from p4rtciface, right?

[pudelkoM]

I moved it back. Let's keep breakage here to a minimum, we can tackle that later.

[ccascone]

We are already introducing a number of breaking changes with Aether 2.0 that will require reviewing config files for existing deployments. This is a good time to clean up the JSON schema.

[osinstom]

LGTM but CI won't allow us to merge this with integration tests failing.