Improve handling of spark-cassandra-stress dropdown value

[nastra]

I improved the handling of the spark-cassandra-stress dropdown value. The initial approach of adding that value (by doing $(e).append($("<option value='spark_cassandra_stress'>spark-cassandra-stress</option>"))) in case dse is selected in the product list lead to a couple of issues, where e.g. the spark-cassandra-stress option wasn't automatically added when the job was cloned or when a new operation was added.

I created a separate function that deals with the spark-cassandra-stress dropdown value. The logic is basically as following:

• in case only a single revision has cassandra and not dse selected, we hide the spark-cassandra-stress dropdown value in all operations dropdown containers
• we show the spark-cassandra-stress dropdown value only when all revisions have dse selected
• in case spark-cassandra-stress is currently selected and we need to hide it, we unselect it and instead select stress from the operation dropdown. Afterwards we hide spark-cassandra-stress from the dropdown

The function maybe_show_spark_cassandra_stress_option() is now called in three cases:

• when the product value is changed
• when a new operation is added
• when a new revision is added

[aboudreault]

LGTM!

[nastra]

thanks for the fast review!