Fixes INFRA-197

The interface name shall start with the hostname of the host it is
associated to.

Note that this is only enforced via the UI (from which most users will interact),
not from the API.

Fixes INFRA-195

- add domain_id on NetworkScope table to define a default domain
- add domain_id on Network (default to the Network Scope one)

Fixes INFRA-194

Some tags (like gateway) should only be selectable by admin users.

One should be able to enter a MAC when creating/editing an interface.

Allow to create/delete cnames when creating/deleting interfaces

Move regex to validators.py

To check if the unique object is the one being edited, we need to store
the object used to create the form in the form (_obj).
This is inspired from flask-admin.

When registering a new host, only admin users shall be able to select
networks marked as "admin_only"

A gateway should be an interface like any other IP.
Use tag to identify it.

Use endpoint argument to avoid collision with flask-admin Network

If a selectfield is disabled, None is returned (not a string).
To pass wtforms validation, that value shall be part of the choices.

We use None as the value for SelectField that are optional (instead of
'').
We were previously converting '' to None when creating a model.
We now use the coerce function to let the form convert 'None' to None.

main blueprint splitted in:
- inventory
- networks

- refactor Network class

This can be done via the Admin interface but this latter shall be
restricted to admin users. This simple page allows the group "create"
to easily add new Manufacturer, Model or Location.