cargo: add feature flag which disables nmstate by default

[prestist]

With the latest release for f42 dependencies which are required for nmstate will no longer be available. There is a PR which has landed https://github.com/nmstate/nmstate/pull/2820 but a build of nmstate needs to land with those changes in f42.

To prevent issues for coreos-installer add a featureflag to make nmstate dependency an optional one. If the feature 'use-nmstate' is not available coreos-installer will lose functionality around customizing network settings. By adding the feature 'use-nmstate' functionality would be restored.