Add RDS/DescribeOptionGroupOptions.
Add a test case for RDS/DescribeOptionGroupOptions.
Add RDS/ModifyDBSubnetGroup.
Add a test case for RDS/ModifyDBSubnetGroup.
Add RDS/ModifyDBInstance.
Add RDS/AuthorizeDBSecurityGroupIngress and some bug fix.
Add RDS/RevokeDBSecurityGroupIngress.
Add a test case for RDS/ModifyDBInstance.
Add a test case for RDS/{authorize,revoke}DBSecurityGroupIngress.