support standard tags

[syalam]

The current community module only tags envtype and envname on select resources. Most terraform resources have a tag {} that allow developers to pass in tags of their choosing. There are N number of cases where more tags need to be applied.

See our fork: https://github.com/blastmotion/terraform-aws-aurora

This module only tags envtype and envname on select resources. I made a backwards compatible change that allows developers to pass in their own set of standard tags which then gets merged with the default envtype and envname tags.

Tags are also applied to any RDS resource that supports them. This way all related resources are tagged appropriately.