pytest --direct appears to be broken

See https://github.com/iotile/coretools/blob/master/iotiletest/iotile/fixture/device_fixture.py#L29

The `--direct` option is specified using `store_true` but it appears that the fixture expects it to contain a `connection_string`.

My understanding is that we no longer use `--direct` anyway so we should deprecate and remove it.

@x24ling @ekkizogloy Do either of you use `--direct` from pytest for anything? If not we should tag it for removal.