Added support for passing flags in override command.

Calling the mesos executor with "./mesos-executor --override ./foo
--bar" would cause the executor flags parsing to fail as it would
continue evaluating arguments as potential flags candidates.  r21388
and r21384 adds the option to use a "--" terminator, which will cause
the mesos flag parsing to stop and modify argv to contain the
remaining arguments.

Review: https://reviews.apache.org/r/21408