Disable ssl test for staging server if current classpath contains the jetty shaded classes

[echarles]

What changes were proposed in this pull request?

The ResourceStagingServerSuite.Enable SSL on the server will will run only if the current classpath does not contain the shaded jetty classes.

If you rely on the spark-core.jar in your classpath, this test will not run.
If you rely on the spark-core module (without shading), this test will be run.

This is to address issues discussed in #463.

To allow test with spark-core.jar, we also add the jetty deps in the kubernetes/core pom (see also #570).

How was this patch tested?

Run maven test phase from the local spark/core module or from the top of the spark maven hierarchy.

[mccheah]

use Utils.classForName, it's the reason why Jenkins is failing the build here.

[echarles]

@mccheah I have pushed an update using Utils.classForName instead of Class.forName to avoid scalacheckstyle error. Let's see what Jenkins tells us.

[echarles]

@mccheah build is Successfull, but showed as Failed due to a Jenkins post-build action to update the Github PR

Could not update commit status of the Pull Request on GitHub.
...
Caused by: com.fasterxml.jackson.core.JsonParseException: Numeric value (4480632093) out of range of int
 at [Source: java.io.StringReader@2b9cc297; line: 1, column: 130]

[ssuchter]

retest this please