- 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

The string representation of a Network is the vlan_name.
This is what we should allow to pass as string to create an interface.

Note that if we pass network_id instead of network when creating
and interface, the validate_interfaces method from the Network class
is not called.

All endpoints are plural

Having the description field only on the Model class makes things
inconsistent.
It's nice to have a description on other attributes as well.

Required to create users when importing items from JIRA.

Renamed blueprint from "users" to "user" to be consistent with other
blueprints (inventory and network).

+ fix tests

- Several IPs (hosts) can be associated to one MAC address
- Host name shall be non nullable

- refactor Network class

- network name should be unique and not nullable
- force all name fields to StringField

Don't ovveride SQLAlchemy __init__ method on db.Model classes.

See http://flask-sqlalchemy.pocoo.org/2.3/quickstart/:
SQLAlchemy adds an implicit constructor to all model classes which
accepts keyword arguments for all its columns and relationships. If you
decide to override the constructor for any reason, make sure to keep
accepting **kwargs and call the super constructor with those **kwargs to
preserve this behavior

Use full exception error as message (error messages from psycopg2 can
be quite long but that makes it much more explicit)

TODO: we should maybe raise ValidationError in models
and catch that to raise CSEntryError in the API

The database constraints will catch the same error
(and raise an IntegrityError).

Having some validation at the model level allows to display nicer error
messages.
We use CSEntryError exception and not ValidationError so that it can be
used both by the API and views.

- Allow to pass several mandatory fields in create_generic_model
- Add tests

- catch IntegrityError on patch
- disable autoflush (IntegrityError was raised before commit due to
autoflush)

Pass ?qrcode=true as query string

Can use:

  -  /api/items?serial_number=123456
  -  /api/items?location_id=1

- add more tests
- patching an item update children location and status
- setting the parent on an item updates the location and status with the
  parent ones

Put under the functional directory all tests that require
access to the database. There is a "session" fixture in that
directory that is automatically invoked for each tests. It rollbacks
any transaction after the test to leave the database clean.

Other tests should go under the unit directory.

Inspired from:
- https://stackoverflow.com/a/38626139
- http://alexmic.net/flask-sqlalchemy-pytest/