Add validation that 'sudo' works for a given user inside of the agitator scripts

[Manno15]

Original Jira ticket: https://issues.apache.org/jira/browse/ACCUMULO-1982

Main snippet:

It would be nice if we tested that sudoing to the desired user worked 
for the current user and gave better error messages in the case of failure.

The code here has changed significantly since the original ticket was created so it is possible the need for this has lessened but from my quick investigation I did not find any validation checks.