Add taskCount and Parallelism options when using dsub with Google Cloud Batch

[croninjoseph]

Google Cloud Batch has the option to set a taskCount and parallelism.
If for example, taskCount was set to 10 and parallelism was set to 2, Batch would run 10 tasks on 2 VMs. This feature would be helpful if available through dsub.

[wnojopra]

The next dsub update is focusing on bringing the google-batch provider closer to feature parity with the other providers. Adding support for setting taskCount and parallelism should be a fairly reasonable feature to support on top of that.

There are a few examples of setting both in the Google Cloud Batch API in their docs. It should be pretty straightforward to add --taskCount and --parallelism options to dsub only for google-batch provider. The only thing to be careful of is that it doesn't interfere with the current --tasks parameter. Technically it should just work, but we'll need to document carefully parallelism across multiple VMs vs parallelism on one VM.