## What changes were proposed in this pull request?

GBTs in pyspark previously had seed parameters, but they could not be passed as keyword arguments through the class constructor. This patch adds seed as a keyword argument and also sets default value.

## How was this patch tested?

Doc tests were updated to pass a random seed through the GBTClassifier and GBTRegressor constructors.

Author: sethah <seth.hendrickson16@gmail.com>

Closes #11944 from sethah/SPARK-14107.