Make sure NetworkManager is running

[todoleza]

Currently the code ignores NetworkManager state completely. This is not expected default environment and should be asserted and fixed before finishing fwdSetup.
Possible solutions:

1. only log beakerlib warning
2. fail in setup if NM is not running -> cause test overall Warn state
3. any of the above and start NM prior finishing fwdSetup

After considering the above, it'd be best to assert that NetworkManager is running in setup phase and if not, start it and keep it running after fwdCleanup as well. This should be possible to tune via environment variables that would allow to:

• ignore NM state altogether (only report to log)
• actively disable NM (and restore to previous state if above variable was set as well)