AWS deployment requires terraform >= 0.12

[sokada1221]

Bug Report

What version of Kubernetes are you using?
N/A

What version of TiDB Operator are you using?
latest master

What storage classes exist in the Kubernetes cluster and what are used for PD/TiKV pods?
N/A

What's the status of the TiDB cluster pods?
N/A

What did you do?

1. Checkout latest tidb-operator
2. terraform apply from deploy/aws

What did you expect to see?
Successful deployment according to the README
terraform version requirement should be documented

What did you see instead?
Parsing error from terraform due to first-class expression syntax. For example, in deploy/aws/bastion.tf, following lines will not work with terraform versions v0.11 and lower:

  vpc_id      = var.create_vpc ? module.vpc.vpc_id : var.vpc_id

  instance_count              = var.create_bastion ? 1 : 0

Looks like this feature was introduced in terraform v0.12.

[sokada1221]

@aylei FYI - seems like a new requirement from #616

Edit: Quickly updated the README to reflect the current state. I personally think this new requirement is okay but I'll leave the design decision on whether older terraform versions should be supported or not to PingCAP.

[gregwebs]

closed by #636