vyos.configdict(): T4228: fix bonding error message interface name

This fixes the error message:

Can not add interface "eth1" to bond, it is already a member of bridge
"{'br6327': {}}"!

As the dict should be moved to a string (br6327)

src/conf_mode/interfaces-bonding.py

@@ -195,11 +195,11 @@ def verify(bond):
                 raise ConfigError(error_msg + 'it does not exist!')
 
             if 'is_bridge_member' in interface_config:
-                tmp = interface_config['is_bridge_member']
+                tmp = next(iter(interface_config['is_bridge_member']))
                 raise ConfigError(error_msg + f'it is already a member of bridge "{tmp}"!')
 
             if 'is_bond_member' in interface_config:
-                tmp = interface_config['is_bond_member']
+                tmp = next(iter(interface_config['is_bond_member']))
                 raise ConfigError(error_msg + f'it is already a member of bond "{tmp}"!')
 
             if 'is_source_interface' in interface_config: