Add capability to define a second set of workers with a different machine type

[trajano]

The idea of it is to have a mix of ARM and x86 based workers to do the work. Since ARM machines cost less.

The general idea would be to deprecate workers and worker_instance_type to end up being

worker_instance_types = [
't3a.medium',
't4g.medium',
't3a.large'
]