'Lists existing Security Groups.
@note: This is a non-standard extension API, and only works for EC2.
:rtype: ``list`` of ``str``'

def ex_list_security_groups(self):

    params = {'Action': 'DescribeSecurityGroups'}
    response = self.connection.request(self.path, params=params).object
    groups = []
    for group in findall(element=response, xpath='securityGroupInfo/item', namespace=NAMESPACE):
        name = findtext(element=group, xpath='groupName', namespace=NAMESPACE)
        groups.append(name)
    return groups
