[SPARK-6402][DOC] - Remove some refererences to shark in docs and ec2

EC2 script and job scheduling documentation still refered to Shark.
I removed these references.

I also removed a remaining `SHARK_VERSION` variable from `ec2-variables.sh`.

Author: Pierre Borckmans <pierre.borckmans@realimpactanalytics.com>

Closes #5083 from pierre-borckmans/remove_refererences_to_shark_in_docs and squashes the following commits:

4e90ffc [Pierre Borckmans] Removed deprecated SHARK_VERSION
caea407 [Pierre Borckmans] Remove shark reference from ec2 script doc
196c744 [Pierre Borckmans] Removed references to Shark