Ballista: Make shuffle partitions configurable

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
When running the benchmarks, I would like to be able to specify the default number of shuffle partitions, rather than have it hard-coded.

Describe the solution you'd like
Introduce a new configuration struct that can be passed to the constructor for a BallistaContext and propagate the settings to the scheduler when executing a query.

Describe alternatives you've considered
None

Additional context
None