fix: elasticache_subnet_group creation

[flightlesstux]

what

• If we pass elasticache_subnet_group_name, the aws_elasticache_subnet_group.default[0] won't be created anymore

why

• Who needs a new elasticache_subnet_group even we already created before and just want to pass a name

references

• Check issue aws_elasticache_subnet_group.default[0] will be created even if we pass elasticache_subnet_group_name #78

[gberenice]

/terratest

[gberenice]

This follows the same logic as we have in terraform-aws-elasticache-redis, which makes sense to me.