vethsetup: make nm_plugins_keyfile consistent

File /tmp/nm_plugin_keyfiles was set only in 03_configure_networking.sh, however repeated run of vethsetup.sh check with cleaned /tmp/ was broken then. Add check to vethsetup.sh.

Also, simplify 03_configure_networking.sh so it no longer needs to check the actve plugins.

Do not exit in vethsetup, use return, as file can be then easily sourced and functions executed without dropping the shell.