T6272: Changed interface existence verification in pppoe/ipoe to Warning

Throwing Warning message instead of Error if interface which is
used in pppoe/ipoe does not exist.

(cherry picked from commit af7277c)

src/conf_mode/service_ipoe-server.py

@@ -66,7 +66,7 @@ def verify(ipoe):
         raise ConfigError('No IPoE interface configured')
 
     for interface, iface_config in ipoe['interface'].items():
-        verify_interface_exists(interface)
+        verify_interface_exists(interface, warning_only=True)
         if 'client_subnet' in iface_config and 'vlan' in iface_config:
             raise ConfigError('Option "client-subnet" and "vlan" are mutually exclusive, '
                               'use "client-ip-pool" instead!')

src/conf_mode/service_pppoe-server.py

@@ -105,7 +105,7 @@ def verify(pppoe):
 
     # Check is interface exists in the system
     for interface in pppoe['interface']:
-        verify_interface_exists(interface)
+        verify_interface_exists(interface, warning_only=True)
 
     return None