Adding support for Mesos shell and arguments parameters to CommandInfo

As per http://mesos.apache.org/documentation/latest/upgrades/, Mesos now supports two modes of command execution - shell or non-shell.  For Docker containers, shell mode means that the Docker container's ENTRYPOINT gets overridden (see https://issues.apache.org/jira/browse/MESOS-1770).  In order to be able to use the standard ENTRYPOINT but override the CMD, we needed to be able to set shell=false on the Mesos task.