Rds refactor

Using rds2 allows tags and the control over whether or not DBs are
publicly accessible.
Move RDS towards a pair of interfaces implementing the details of rds
and rds2
Added tests to ensure that all operations work correctly as well as
requirements files that allow virtualenvs to test either boto.rds or
boto.rds2

Replaces the modules component of ansible/ansible#8834

Tests will continue to be in ansible/ansible#8834