[SPARK-10749][MESOS] Support multiple roles with mesos cluster mode.

Currently the Mesos cluster dispatcher is not using offers from multiple roles correctly, as it simply aggregates all the offers resource values into one, but doesn't apply them correctly before calling the driver as Mesos needs the resources from the offers to be specified which role it originally belongs to. Multiple roles is already supported with fine/coarse grain scheduler, so porting that logic here to the cluster scheduler.

https://issues.apache.org/jira/browse/SPARK-10749

Author: Timothy Chen <tnachen@gmail.com>

Closes #8872 from tnachen/cluster_multi_roles.