add xgboost server rock and tests

[ca-scribner]

Closes #26.

Testing instructions:

See xgboost/readme.md

[orfeas-k]

Everything looks good. I left two small comments.

Btw, the only difference in response I noticed when testing is

-  "model_version": "v1.0.0",
+  "model_version": null,
+  "parameters": null,

We 've seen this happen in other places where we use ROCKs like canonical/seldonio-rocks#71 and I 'm still wondering why, but yeah it's not a blocker. I just thought of mentioning it.

[orfeas-k]

Also, it would be nice if we had the testing instructions automated as sanity tests.

[ca-scribner]

@orfeas-k agreed about the testing instructions. I actually looked into it, but didn't find a simple way so I dropped it for now. The thing I was hung up on was trying to do it from tox, and I wasn't sure how to docker run the thing and then curl at it. But it would help a lot if they were automated - I just added #47 to capture this

[orfeas-k]

thank you @ca-scribner