Add some useful environment variables to the Chronos task

This adds the following environment variables and their purpose

CHRONOS_RSRC_CPU  - The CPU resources allocated to this job
CHRONOS_RSRC_DISK - The disk resources allocated to this job
CHRONOS_RSRC_MEM  - Tme memory resources allocated to this job

The value in these is certain programs (like the JVM) can use these
values to optimize how they run (like setting the -xmx parameter).
Honestly Mesos should do this but I figure it doesn't hurt to put
it here and later have Mesos provide it too.

@mngo87  Please review...