some tests for the new mon_errors check

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
create a helper for a safe get on ConfigParser

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
add a check to catch common monitor deployment errors

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>