Added the new API v2 hostname service

[teclator]

Problem

Agama allows to set the hostname during the installation. We need to expose this functionality using the new API and adapt the UI accordingly.

• https://trello.com/c/tuQh4U24/5445-adapt-the-hostname-api-to-the-new-approach

Solution

Added hostname service to the backend. The UI has been adapted allowing to set the hostname using the new API but the registration check is missing because product registration is still not adapted in the UI (see #2946 for example).

Todo

• New hostname service is defined
• UI is adapted (registration check is pending until implemented)

Testing

• Tested manually

[imobachgs]

The approach looks good, although I know that some things are still missing.

[imobachgs]

np: If you want, you could implement something like:

Config::new()
  with_hostname("test");