Add support for overridding the number of queries in the workload

[daverigby]

To allow users to simulate workloads which are similar to existing (but not necessarily identical), it would be valuable to be able to override the number of queries which will be issued via the command-line - say via a --queries=N option.

This should be passed to the Workload being instantiated, which should then modify it's behaviour to use the given number of queries.

(Note: similar to #183).