Do not set MIG target_size when auto scaler is enabled

Fix issue/27 target_size should not be set with an auto scaler.

Pin the OS image used to reduce risk

• Pin the os image to a specific version to ensure consistent behavior when scaling in, scaling out, auto-healing, and across multiple terraform apply runs.
• Replaced the os_image input var with image_project, image_family, and image_name.

Zero downtime upgrades

v3.0.0 - 2020-09-30

• Add support and documentation for zero-downtime upgrades. See
  issue/23.
• Follow the process described in UPGRADE.md when
  upgrading to this major version.

Deploy to all available zones in the region

v2.0.0 - 2020-09-29

• Fix issue/20 modules/52_regional_multinic now deploys instance
  groups to all available zones in the specified region. Fixes error
  deploying to us-east1 and europe-west1 where there is no a zone.
• Note, resources will be destroyed and re-created. The inputs to
  52_regional_multinic have not changed relative to v1.4.0. The zone
  input to 50_compute is replaced by zones.