Construct an http client suitable for googleapiclient usage w/ user agent.

def _build_http(http=None):
    """Construct an http client suitable for googleapiclient usage w/ user agent.
    """
    if not http:
        http = httplib2.Http(
            timeout=HTTP_REQUEST_TIMEOUT, ca_certs=HTTPLIB_CA_BUNDLE)

    user_agent = 'Python-httplib2/{} (gzip), {}/{}'.format(
        httplib2.__version__,
        'custodian-gcp',
        '0.1')
    return set_user_agent(http, user_agent)