feat: add server readiness elb health checks configuration options

[joelmccoy]

Fixes #105 .

This PR adds in configuration options to the terraform module so that we can have better health checks for the rke2 server nodes. It introduces the ability to set the server nodes autoscaling health check to ELB (ref).

Also added additional configuration options:

• enforce_min_elb_capcity which checks to make sure all server nodes are up before terraform proceeds
• server_node_health_check_grace_period allows you to override the default 300s before considering an instance healthy
• wait_for_elb_capacity_on_updates enables enforce_min_elb_capacity for autoscaling group updates (and not just initial creation)

All these changes are added in a nonbreaking change manner (default values are set to originals).