Allow custom TFE endpoint for --tfe-token credential block

[mechastorm]

Currently when using --tfe-token or ATLANTIS_TFE_TOKEN environment variable, Atlantis will create a credential block in the .terraform

credential "app.terraform.io" {
}

It would be good if we have an option to override the value app.terraform.io. This is to enable support for privately hosted TFE installations that have a custom endpoint url.

Current workaround is to not set the TFE token in the Atlantis runtime and statically define the .terraformrc file