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).