Implement settings modification using the command line flags, instead of manually editing the config files

[yaroslav-gwit]

As Hoster matures and our user base grows, it becomes harder to support a current model of "it's easy, just edit the config file".

Step-by-step, I would like to implement command line flags that modify certain (most used, for now) config parameters. For example, to add a new network you could simply call:

hoster network add --name wan1 --apply-bridge-address --network-gateway 10.0.101.254

instead of adding this to the config file manually:

    {
        "network_name": "internal",
        "network_gateway": "10.0.101.254",
        "network_subnet": "10.0.101.0/24",
        "network_range_start": "10.0.101.10",
        "network_range_end": "10.0.101.200",
        "bridge_interface": "None",
        "apply_bridge_address": true,
        "comment": "Internal Network"
    }

which is not only error prone, but also hard for users who aren't familiar with JSON.

The same could be applied to the ZFS datasets, adding new networks to VMs, mounting ISOs, etc etc.

For the advanced users out there - JSON config files aren't going anywhere, and you'll still be able to edit them by hand, or maintain them via Ansible and other automation tools. This change is purely here to make life easier for the new users, and one-off changes.