[SW-683] Fix starting H2OContext on Databricks

[jakubhava]

We are using internal Spark API available inside package org.spark_project.jetty to register new tabs, however this API(class) is not available on Databricks - they are using regular jetty for this in org.eclipse.jetty (this is even more complicated by the fact that we are shadowing the jetty)

We have several options here:

1. Add also org.spark_project.jetty to assembly jar and Sparkling Water core dependencies so it's available on the Databricks and in

   sparkling-water/core/src/main/scala/org/apache/spark/h2o/ui/SparklingWaterInfoPage.scala

   Line 171 in 0c19d24

   private def attachHandler(): Unit = {

   create actually 2 variants of methods attachHandler where one would use the method from regular jetty and one from spark jetty and they would be invoked based on the current environment ( Databricks or not)

Also, we couldn't use features for downloading full and partial logs in case of Databricks since it's again using different paths and API for these handlers. So we can again disable obtaining logs on Databricks.

I have this PR prepared and tested, we can go with this variant, however, I prefer the version implemented in this PR which is 2)

2. The second option is to simply not add Sparkling Water UI tab in case of Databricks.

The second option is definitely cleaner regarding the code and dependencies in spark assembly ( since we need to provide org.apache.spark-core to make those org.spark_project classes available and these classes might collide with the same classes in some environment`

The first option is more ugly, however, gives at least basic Sparkling Water UI to DBC users.

I still prefer the simple variant implemented in this PR. We can always implement the first when there is demand for it until then, keep the code cleaner.

What do you think @mmalohlava, please?

[rgabo]

@jakubhava we've run into the exact same issue last night. I'm glad to see that a PR is available to address the issue. We can build and test the fix ourselves and let you know.

[jakubhava]

Let me merge this change as it's always easy to implement the second approach. Right now this will just unblock the DBC users in a easy way