dynamically name rds subnet group
dynamically name rds subnet group

It appears that rds subnet group names need to be unique on a per
account/region basis.  A hard coded subnet group name prevents multiple
identical VPCs in the same region.

jhoblitt/sandbox-jenkins-demo,lsst-sqre/sandbox-jenkins-demo,jhoblitt/sandbox-jenkins-demo,lsst-sqre/sandbox-jenkins-demo