Reportal should let user kill his or her running report

[erwa]

Sometimes after firing off a run, I realize there is a bug in my code. I would like to kill the running report to avoid wasting resources. Currently, there is no way for me to kill the report from the Reportal UI. We should add this functionality.

This is especially critical now that Reportal only allows you to have one RUNNING execution of each report at a time. Sometimes jobs have OOM errors and hang, causing the flow to remain in the RUNNING state forever. In such a scenario, I will never be able to run my report again, since I have no way of killing the currently RUNNING execution.

Alternatively, we should roll back this commit, or make a new commit so that concurrent execution can be enabled/disabled by a property in the Azkaban conf file.

Tracked by internal JIRA HADOOP-6976.