Move dbinstance into a TemplateResource

Move dbinstance into a TemplateResource

The old dbinstance will soon be overtaken by the trove resource.
- The only reason for keeping this around is for people that don't
  have trove installed and want to use the AWS resource.
- Seperating it out into a TemplateResource really makes it easier
  for deployers and users to customise it.
- The old dbinstance did nothing "special" in python, and was really
  one of the first "TemplateResources" as it attempted to convert properties
  into parameters. Since this in now done a lot better in the TemplateResource
  lets just make use of that.

This will make it easier to migrate to other distros and versions.

Partial-Bug: #1215797
Change-Id: If72e1f40f67dc831551e0db8df8caaa002aaaeda