Simplify deployment; Use `vault_hostname` instead of `vault_aws_certificate_arn`

[robertdebock]

The module now required the variable vault_aws_certificate_arn to be set. To make a vault_aws_certificate_arn, the examples/*/main.tf create a few resources.

The resource vault_aws_certificate_arn is somewhat difficult to explain, whereas vault_hostname seems much more understandable.

Proposal:

• Make vault_aws_certificate_arn optional. (default = "")
• Introduce a new variable vault_hostname (or so).
• Include these resources in the module: aws_acm_certificate, data: aws_route53_zone, aws_route53_record and aws_route53_record.

If a vault_aws_certificate_arn is provided, simply use it in the module. If vault_aws_certificate_arn is not provided, create the required resources.

[robertdebock]

Still relevant; either

• Have a domain/hostname, the module will create certificates.
• Bring your own certificate, the module will skip all certificate steps.

This would require a major version release.