Enforce eager validation of configuration parameters

This leads to a lazy validation approach, which is problematic, as once the gateway is initialized there's no way to fix configuration problems (the config file is read once at initialization phase).

This should be generally aligned with a 'fail-fast' approach refactoring. That is, fail ASAP if:
* config parameters are invalid,
* Ceph auth permissions are incorrect (right now there are situations where the gateway starts without proper permissions and only fails when it tries to write something to the RADOS state object).

Suggestions to fix this:
* Enforce eager validation:
* This has already been tried with home-brew approach: https://github.com/ceph/ceph-nvmeof/pull/126.
* Using existing libraries, like [`configargparse`](https://github.com/bw2/ConfigArgParse), which leverages `argparse` syntax to enforce preemptive validation.
* Define a schema for the config file:
* Given `configparse` INI files can be easily converted into dicts, and dicts into JSON, we could easily use JSON Schema to perform a validation. However that only covers validation issues [\#1](#param_not_defined) and [\#2](#param_not_correct), and JSON Schema is not the friendliest thing to work with (and doesn't enforce configuration-as-code pattern either).