'Remove an existing rule from an existing security group.
You need to pass in either ec2_security_group_name and
ec2_security_group_owner_id OR a CIDR block.
:type group_name: string
:param group_name: The name of the security group you are removing
the rule from.
:type ec2_security_group_name: string
:param ec2_security_group_name: The name of the EC2 security group
from which you are removing access.
:type ec2_security_group_owner_id: string
:param ec2_security_group_owner_id: The ID of the owner of the EC2
security from which you are
removing access.
:type cidr_ip: string
:param cidr_ip: The CIDR block from which you are removing access.
See http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing
:rtype: bool
:return: True if successful.'

def revoke_dbsecurity_group(self, group_name, ec2_security_group_name=None, ec2_security_group_owner_id=None, cidr_ip=None):

    params = {'DBSecurityGroupName': group_name}
    if ec2_security_group_name:
        params['EC2SecurityGroupName'] = ec2_security_group_name
    if ec2_security_group_owner_id:
        params['EC2SecurityGroupOwnerId'] = ec2_security_group_owner_id
    if cidr_ip:
        params['CIDRIP'] = cidr_ip
    return self.get_object('RevokeDBSecurityGroupIngress', params, DBSecurityGroup)
