option to introduce multiple instances with redis locking

[nitrocode]

Redis locking would allow an HA setup using multiple atlantis servers.

https://www.runatlantis.io/docs/server-configuration.html#locking-db-type

Related issue terraform-aws-modules/terraform-aws-atlantis#322

CloudMemory store would be the redis equivalent of aws elasticache

This module would not need to implement the redis cluster itself and it may not need adding a count to the GCE instance since the count could be added to the module itself.

An example with a CloudMemory instance with its values fed into this module would be enough to allow people to use it

resource "google_redis_instance" "cache" {
  name           = "memory-cache"
  memory_size_gb = 1
}

module "atlantis" {
  source     = "bschaatsbergen/atlantis/gce"

  count = 4

  name = "atlantis-${count.index}"

  env_vars = {
    ATLANTIS_LOCKING_DB_TYPE = "redis"
    ATLANTIS_REDIS_HOST      = ""
    ATLANTIS_REDIS_PORT      = ""

    # could be randomly generated when the redis cluster is born
    ATLANTIS_REDIS_PASSWORD  = ""

    # ...
  }

  # ...
}