Reorganize configuration of the http server building

rework the configuration used by `make_app()` so that one can start the
http server usig an existing grpc server by a setting the configuration
file accordingly.

This make the `make_app()` function only accept one config structure
(dict), which structure would be:

  graph:
    cls: local
    grpc_server: # config for spawn_java_grpc_server()
      ...
    http_rpc_server: # config for GraphServerApp()
      ...

or:

  graph:
    cls: remote
    url: # host:port of the running grpc server
      ...
    http_rpc_server: # config for GraphServerApp()
      ...

This also comes wit a few other refactorings:

- rename swh/graph/rpc/ as swh/graph/grpc/ to prevent confusion with the
  (http) rpc server.
- similarly, rename swh/graph/rpc_server.py as swh/graph/grpc_server.py
- and rename swh/graph/http_server.py as swh/graph/http_rpc_server.py
- remove the "graph" config section from the grpc_server config
  structure (it had only the "path" config entry).