netcontrol/api.py: Move Device to core/network.py

`api.py` needs to use Device only to expose it in the API, whereas other
modules like `network.py` will make more extensive use of it. Since
`api.py` is highly likely to import most of the low-level modules, it's
better that they are able to import it from another module different than
api.py. Otherwise, it would be extremly easy to get circular imports.
extra/pylint.conf: Disable R0903 "Too few public methods"

This was in:

netcontrol/core/network.py:35: [R0903, Device] Too few public methods (0/2)
network.Device: Rename arguments to `ip_address` and `mac_address`

To have a good naming consistency along all the code files, because it was
getting a bit confusing.