Support AWS Access Keys in retry_join

Background:
The Consul auto-join feature supports access-keys to discover nodes it should join. Ideally you should be using a role that has the describe-instance permission attached to it. This works great if your cluster is entirely within a single account.

However, there is currently no way use the auto join feature across AWS accounts with IAM roles. The solution for now is to use a AWS access keys to be able to describe instances in a different account.

Problem:
The run-consul script does not support access keys.

terraform-aws-consul/modules/run-consul/run-consul

Line 253 in db3d81e

    
           "retry_join": ["provider=aws region=$instance_region tag_key=$cluster_tag_key tag_value=$cluster_tag_value"],

Solution
Add the ability to set both access_key_id and secret_access_key to the retry_join block as mentioned here: https://www.consul.io/docs/agent/cloud-auto-join.html#amazon-ec2

Since this isn't a super common use case, is it something you could add to your own config by overriding the configuration?