Edit default value for availability_zones and subnet_ids vars to null

[rafaelremondes]

Related to Issue 181 in terraform-aws-consul.

Newest provider-aws version only allows us to set one of the 2 variables: availability_zones and subnet_ids when creating AutoScalingGroups.

As per @brikis98 suggestion, I am changing the default value of these 2 variables to null so they're no longer conflicting, allowing to set either of them when using the vault_cluster module.

[hashicorp-cla]

All committers have signed the CLA.

[brikis98]

Thanks for the PR! We need to resolve #196 first and can then take a look at this PR. Hoping to get to that in the next week or two. If anyone has cycles to look into it before then, help is very appreciated!

[brikis98]

Thanks for the PR! This was already done as part of the fixes in #195.