Refactor integration contexts for less duplication

The `auto_network 0.x` and `auto_network 1.x` contexts both included a bunch of
duplicated setup code. This shared code has been moved into the
`isolated_settings` context which has been renamed to:

    auto_network integration

Also moved the `current_ip` helper into the `auto_network integration` context.
Add integration tests for the "request" action

Replaced pending integration test with two tests that exercise
`AutoNetwork::Action::Request` under the following conditions:

  - Machine created and no IP is allocated.

  - Machine created and an IP is pre-allocated.

The second test is currently failing due to a bug.

Ref. GH-23.
Handle pre-allocated IPs in the Request action

This patch fixes a bug that occurred when a newly created VM already had an IP
allocated in the AutoNetwork pool. The `FilterNetworks` action would not fire
because the VM was not created. The `Request` action would not fire because an
IP had already been allocated.

This patch extends the `Request` action such that it ensures no unfiltered
AutoNetwork networks remain on a VM in addition to allocating an IP if
necessary.

Fixes GH-23.