Implement Configurable Instance Number in Simulations for gPBFT Package

[masih]

Simulations always start at instance zero. Most tests also implicitly assume this. But the gPBFT package allows a user to start at a non-zero instance.

Implement the ability to configure instance number in simulations and add additional tests to exercise relevant code.

[anorth]

I think this was posted in response to a comment I made in code review, but I don't think this is worth doing.