Define 'type' in argument_spec of openvswitch_port (#2355)

The external_ids 'type' was not defined in the argument spec of
openvswitch_port. This lead 'external_ids' to be converted to a
string, when the value was not defined. Further down the code this
was leading to an exception in some cases. By defining the type
all is right.

network/openvswitch_port.py

@@ -238,7 +238,7 @@ def main():
             'state': {'default': 'present', 'choices': ['present', 'absent']},
             'timeout': {'default': 5, 'type': 'int'},
             'set': {'required': False, 'default': None},
-            'external_ids': {'default': {}, 'required': False},
+            'external_ids': {'default': {}, 'required': False, 'type': 'dict'},
         },
         supports_check_mode=True,
     )