Improve usage of MagicMocks in ML2 and L3 tests

Removes usage of a MagicMock in a case where it's not needed.
Replaces a manual function replacement with a mock.patch call
so it's correctly cleaned up by mock.patch.stopall.

Partial-Bug: #1316401
Change-Id: I8f2f7e9eca2aeaa3b65be1b451e96ed2146950f4
Changed DictModel to dict with attribute access

DictModel is a dict where keys are accessible via attribute access.
The old version was an object with attributes created from dict keys and many
attributes where accessible only via getattr because they did not have a valid
python attribute naming (i.e.: 'provider:network_type' is not a valid
python variable/attribute name)::

    >>> d.provider:network_type
      File "<stdin>", line 1
        d.provider:network_type
              ^
    SyntaxError: invalid syntax

This time we just subclass dict with attribute access to keys.

So dict keys are accessible via attribute access but remain accessible via key
access::

    >>> d = DictModel({'foo': 'bar', 'provider:network_type': 'something'})
    >>> d.foo == d['foo']
    ... True
    >>> getattr(d, 'provider:network_type') == d.get('provider:network_type')
    ... True
    >>> getattr(d, 'provider:network_type') == d['provider:network_type']
    ... True

One of the big advantages when debugging is that now in pdb, pp(d) (where d is
a DictModel instance) shows a nice dictionary structure, while with the old
version whe had to perform a "dir(d)" and introspect each attribute...

Change-Id: I05fad7cd9763f97f61680d45e1b6592a80049541
Closes-Bug: 1251653
Refactor code in update_subnet, splitting into individual methods

The update_subnet was already long and complex enough and adding
more code to it was undesirable, after commit

I47a3a71d0d196b76eda46b1d960193fb60417ba9

Hence this refactor splits parts of the update_subnet code into
dedicated sub-methods.

Change-Id: I8f98938da88db287290493dc0daf7c81b8b5f542
Merge "Refactor code in update_subnet, splitting into individual methods"
Merge "Changed DictModel to dict with attribute access"
Merge "Improve usage of MagicMocks in ML2 and L3 tests"