Define FakeMachine helper for functional/fullstack tests

Define FakeMachine helper for functional/fullstack tests

The change defines the FakeMachine fixture/helper which emulates a
machine through a namespace with:
* a port bound to a bridge,
* an ip on the port,
* a gateway (if requested).

The FakeMachine class can be used to emulate:
* a VM for testing network features (ex: metadata service),
* an external machine for testing "external" network features (ex:
  routing/natting),
* a server for low level tests of network features (ex: iptables).

The change also defines PeerMachines fixture/helper to create some fake
machines bound to a bridge.

Change-Id: I4fde1a03badd9adfd14b9124b5602331b69dda9d