systemd: Allow use of environment file to run NM with custom envvars

Add config to NM service to load environment variables in the file at /etc/NetworkManager/NetworkManager.env. If the file does not exist, which it will not by default, do not error. This is useful easily setting envvars in NM. Specifically, the nm-openvpn plugin can only be configured via specific envvars. There is currently no mechanism for using NM config files to specify envvars for plugins.