v1.0.8: Support subnet-specific additional tags (#51)

* Added var.public_subnet_tags
* Added var.private_subnet_tags
* Added var.database_subnet_tags
* Added var.elasticache_subnet_tags
* Removed Tier subnet tag

This helps solve a specific issue with Kubernetes, where public &
private subnets must be appropriately tagged for public/private ELBs to
work.

v1.0.7: Merge pull request #50 from tfhartmann/adding_network_tier_tags

Adding a Tier tag to all subnets