Make Katib work with Ambassador. (#1103) (#1150)

* Katib uses "/katib" as the route path; so we change the Ambassador mapping
  to use /katib and not /modeldb as the prefix.

* I was able to load /katib/projects successfully behind IAP with this change
  I didn't actually try to use it though.

Fix: kubeflow/katib#118

kubeflow/katib/modeldb.libsonnet

@@ -161,8 +161,8 @@
               "apiVersion: ambassador/v0",
               "kind:  Mapping",
               "name: modeldb-mapping",
-              "prefix: /modeldb/",
-              "rewrite: /",
+              "prefix: /katib/",
+              "rewrite: /katib/",
               "method: GET",
               "service: " + "modeldb-frontend." + namespace + ":3000",
             ]),