Inconsistent prameter name - pod

[Shinobu KINJO]

There are parameters regarding to pod. Naming of this kind of parameter is a little bit different.
It could confused contributors or maintainers. Since that, this inconsistency has to be fixed out as much as possible, and as soon as possible.

e.g.,
./nova_apigw/controllers/server.py: def _get_or_create_route(self, context, pod, _id, _type):
./nova_apigw/controllers/server.py: def _get_client(self, pod_name=constants.TOP):
./nova_apigw/controllers/volume.py: def _get_client(self, pod_name='top'):

Maybe it could prevent this project from growing; meaning that it's really serious.