Provide a scaling factor for mapred.task.timeout inside functional test's RunTests.java

The MR option for running functional tests ($ACCUMULO_HOME/test/system/auto) fails with a mapred timeout currently set at 480 secs (8*60*1000). A flag or argument for scaling mapred.task.timeout inside RunTests.java (ie, a number that influence the 8 in 8*60*1000) needs to be in place to allow scaling up or down mapred.task.timeout as needed. This is similar to ACCUMULO-1829.