'Returns descriptions for existing application versions.
:type application_name: string
:param application_name: If specified, AWS Elastic Beanstalk restricts
the returned descriptions to only include ones that are associated
with the specified application.
:type version_labels: list
:param version_labels: If specified, restricts the returned
descriptions to only include ones that have the specified version
labels.'

def describe_application_versions(self, application_name=None, version_labels=None):

    params = {}
    if application_name:
        params['ApplicationName'] = application_name
    if version_labels:
        self.build_list_params(params, version_labels, 'VersionLabels.member')
    return self._get_response('DescribeApplicationVersions', params)
