Pull kubernetes config keys out into a separate class

[ash211]

Something like the below class for kubernetes would be good for making sure we're not missing config keys in the docs, plus generally pull all config keys into one place which seems like a good idea.

https://github.com/apache/spark/blob/v2.1.0/yarn/src/main/scala/org/apache/spark/deploy/yarn/config.scala

[mccheah]

+1 - I'll take ahold of this, primarily because Client.scala is getting a little cumbersome to read.