simplify the docker build base and runtime images + remove the go.rice trickery (#48)

* adjust the prototype for building oni test plans.

- We no longer clone the entire lotus codebase.
- We only clone and build the ffi module in the build image.
- The runtime image no longer builds the ffi module.
- The runtime image uses go-paramfetch to fetch the parameters
  (which are not expected to change).
- We use filecoin-project/lotus#2097 to not have to deal with
  go.rice trickery for things we don't actually need to use.

* add filecoin-ffi submodule (commit ca281af); fix replace directive.

* remove stale file.

* tag build base images.

* fix build.sh.

* delete filecoin-ffi submodule from root.

* add filecoin-ffi submodule under extra/filecoin-ffi.

* adjust everything to use extra sources TG feature.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "extra/filecoin-ffi"]
+	path = extra/filecoin-ffi
+	url = https://github.com/filecoin-project/filecoin-ffi.git