CLI: write network configuration

[imobachgs]

Problem

#622 added support to display the network configuration through the CLI: agama config show. However, writing the configuration was still missing.

Trello card: https://trello.com/c/vVe8CboU/3354-8-agama-set-the-network-configuration-using-a-jsonnet-file

Solution

This PR implements support to set the network configuration using the CLI. But, as usual, we took the chance to do some refactoring and clean-up.

Note: naming network connections

Agama's network service API works with connection names (Id) instead of UUIDs. In NetworkManager, you can use any of them. However, we prefer our users to work with Ids, not UUIDs, because they are more human-readable. The problem is that, in NetworkManager, the Id is not unique.

This PR modifies Agama to rename duplicated network connections. For instance, if we have multiple connections with the same name (e.g., virbr0), Agama will add a suffix to the duplicates (e.g., virbr0-1, virbr0-2, etc.).

Testing

• Added (a few) unit tests
• Tested manually

Future

We would like to implement a few changes, but we should address them separately.

• Bound a connection to a specific network interface. By now, NetworkManager chooses the device automatically.
• The software service might need to wait until the network is ready.
• Do not update those connections that did not change.

[coveralls]

coverage: 72.515% (-0.3%) from 72.829% when pulling 4c51e6a on imobachgs:cli-write-network into b566243 on openSUSE:master.

[joseivanlopez]

LGTM