List all gear units including dead ones

I've added the `?all=1` to the gear list url and added a test which demonstrates the effect.

But I'm not entirely sure which extra states are included by making that change.

For example `failed` units (eg units created with a non-existent docker image name) are listed *anyway*.

I'm also not sure how units get into the `dead` state. The example in my test (and the example that @itamarst used in https://github.com/ClusterHQ/flocker/issues/208#issuecomment-48231413) recreate the dead state by creating a gear unit using the busybox image which exits immediately with a 0 status code. I'd have thought that systemd would attempt to restart such units a few times before marking them failed, but that doesn't seem to happen.

I also need some guidance around how to wait for the unit-under-test to reach the `dead` state. I can test the current state using `systemctl`, but that won't make any sense when running these tests against the FakeGearClient.
Perhaps this particular test should be move to the implementation specific `flocker.node.functional.test_gear.GearClientTests` case.

Fixes: #208
You should make an assertion about the state as well, to make sure it's testing what you think it's testing (e.g. if it's just very long running this test will pass.)