Increase the min number of instances and the cool down period

[past]

The current webapp VM configuration is:

automatic_scaling:
  cool_down_period: 120s
  min_num_instances: 2
  max_num_instances: 20
  cpu_utilization:
    target_utilization: 0.5

This change removes the instance_class that is only applicable to the standard environment (c.f. flex docs), doubles the minimum number of instances and increases the cool down period to avoid health check errors during instance initialization.

[past]

The GCP recommendation is to overprovision by 50%, but I doubled it just to be safe.