nm: Fix the incorrect change indication when apply the same config twice

When applying the same network connections twice, the second apply still
shows `changed: true`.

The root cause:
 * When user never asked about ethtool configuration, network-role
   will generate an all-default `NM.SettingEthtool` and pass it to
   NetworkManager daemon. But NetworkManager discard it when saving to
   ifcfg plugin as ifcfg plugin will not keep empty ethtool option.

 * During second apply, the `NM.SimpleConnection.compare` will return
   False indicating configuration changed because of on-disk connection
   has no ethtool setting while pending connection does.

To fix it, we just remove the all-default `NM.SettingEthtool`.

Signed-off-by: Gris Ge <fge@redhat.com>

library/network_connections.py

@@ -976,6 +976,16 @@ def connection_create(self, connections, idx, connection_current=None):
                     else:
                         s_ethtool.option_set_uint32(nm_ring, setting)
 
+            # * When user never asked about ethtool configuration, network-role
+            #   will generate an all-default `NM.SettingEthtool` and pass it to
+            #   NetworkManager daemon. But NetworkManager discard it when saving to
+            #   ifcfg plugin as ifcfg plugin will not keep empty ethtool option.
+            # * The follow up `NM.SimpleConnection.compare()` will indicate
+            #   configuration changed as new ethtool setting found.
+            # * To workaround this, we just remove the all-default NM.SettingEthtool.
+            if s_ethtool.compare(NM.SettingEthtool.new(), NM.SettingCompareFlags.EXACT):
+                con.remove_setting(NM.SettingEthtool)
+
         if connection["mtu"]:
             if connection["type"] == "infiniband":
                 s_infiniband = self.connection_ensure_setting(con, NM.SettingInfiniband)