Support standard GRANIAN_* env vars and the rloop event loop

- Launcher now only injects --interface/--host/--port when the matching
  GRANIAN_* env var is unset, so every granian option (GRANIAN_LOOP,
  GRANIAN_WORKERS, GRANIAN_HTTP, GRANIAN_HOST, GRANIAN_PORT, ...) can be
  configured via granian's standard GRANIAN_* environment variables
- Add the rloop event loop as a runtime dependency so GRANIAN_LOOP=rloop
  works out of the box (previously failed with a missing 'rloop' package)

requirements.in

@@ -4,3 +4,4 @@ kaya-core
 aiomcache
 kaya-rsgi
 granian
+rloop