Add requirement to SecurityGroup "remote_group_id"

"remote_group_id" propertie needs to be set only if "remote_mode" is
set to "remote_group_id". If "remote_mode" is in its default value,
remote_group_id is ignored, there is no error message.

There is no clue to understand that remote_mode should be set to
another value, except reading all SecurityGroup properties.
Also, remote_mode does not appear in "neutron security-group-show"
output.

Change-Id: Icdf698a6d7b5539bbc7c4426aa88250229f795f0
Merge "Add requirement to SecurityGroup "remote_group_id""