environment: do not start NM if found stopped
This breaks run_once tests, as the test leaves NM stoped with "configure-and-quit" option enabled.
It is useless to start NM in this case. If it was stopped on purpose
during the test, there must be @restart_if_needed. In case of crash such
that systemctl start NetworkManager
is not able to start NM, it is
usesess either to start it again, before tags cleanup or after crash
reset (where NM config is cleaned and NM restarted).
@Build:nm-1-32 @os:c8s