Add integration test for filter_networks

Most of the changes in this patch involve creating `vagrant-spec` fixtures that
can support complicated integration tests.

All AutoNetwork actions that fire during envrionment initialization are now
covered by integration tests.
Add integration tests for release action

Test that destroying a machine results in an IP address being released from the
pool.
Add pending test for request action

The request action is tough to test as, by necessity, it hooks in everywhere
`Vagrant::Action::Builtin::ConfigValidate` runs. Currently, the dummy provider
available to integration tests isn't sophisticated enough to include this in
action chains.