Allow Dev Mode with new goal network create with --devMode override

[tzaffi]

Problem

Currently network DevMode is specified by choosing a particular network config file with Genesis.DevMode = true. However, since go-algorand PR #3904 one can add this capability to any network chosen. It would be nice to add this option to sandbox.

Solution

One possibility is to add a dev_mode = False parameter to setup.py's create_private_network().

Dependencies

None

Urgency

Medium: this is a nice feature but seems orthogonal to other work. However, the more we can programmatically control a network's features, the easier it will be to bring up networks with desired characteristics and the less the maintenance hassle.