Allow configuring custom releasesURL

Some environments do not have access to releases.hashicorp.com. It would be nice to be able to specify a custom url so that Atlantis would be able to pull from internal mirrors.

https://github.com/runatlantis/atlantis/blob/2f54257b43cc4105027cea40417991c640f77043/server/events/terraform/terraform_client.go#L81