0.78.0 [ecs] Update `atlantis` version. Add `atlantis_repo_config` variable

what

• Update atlantis version
• Add atlantis_repo_config variable
• Remove --allow-repo-config variable

why

• cloudposse-archives/terraform-aws-ecs-atlantis#7
• Flag --allow-repo-config has been deprecated in favor of creating a server-side repo config file that applies the same configuration
• Use a server-side config file and point --repo-config to it

references

• https://github.com/runatlantis/atlantis/releases/tag/v0.7.0
• https://www.runatlantis.io/docs/configuring-atlantis.html
• https://www.runatlantis.io/docs/server-configuration.html
• https://www.runatlantis.io/docs/server-side-repo-config.html
• https://www.runatlantis.io/docs/repo-level-atlantis-yaml.html