changing AWS_SECURITY_TOKEN to AWS_SESSION_TOKEN (#8816)
changing AWS_SECURITY_TOKEN to AWS_SESSION_TOKEN (#8816)

I noticed that Terraform is not (anymore) parsing the value of environment variable `AWS_SECURITY_TOKEN` (which was re-added in May 2015: https://github.com/hashicorp/terraform/pull/1785)

Example ENV:

```
AWS_SECURITY_TOKEN="FQo...vgU="
AWS_ACCESS_KEY_ID="A...A"
AWS_SECRET_ACCESS_KEY="I...t"
AWS_DEFAULT_REGION="eu-west-1"
```
This errors with messages akin to "AWS was not able to validate the provided access credentials".

Setting `AWS_SESSION_TOKEN` instead of `AWS_SECURITY_TOKEN` on the other hand works just fine and seems to be in line with what is suggested in the code: https://github.com/hashicorp/terraform/blob/d1ac7d3b2e1be2f32092275340f33a2d73f7b31a/vendor/github.com/aws/aws-sdk-go/aws/credentials/env_provider.go#L69, making this whole thing a documentation change.

Happy to provide test cases or additional insights though!
Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,ashinohara/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws