'Retrieve a session key from the array.'

@utils.synchronized(__name__, external=True)
def _get_session_key(self):

    self._session_key = None
    hash_ = ('%s_%s' % (self._login, self._password))
    if six.PY3:
        hash_ = hash_.encode('utf-8')
    hash_ = hashlib.md5(hash_)
    digest = hash_.hexdigest()
    url = ((self._base_url + '/login/') + digest)
    try:
        xml = requests.get(url, verify=self.ssl_verify, timeout=30)
    except requests.exceptions.RequestException:
        msg = _('Failed    to    obtain    MC    session    key')
        LOG.exception(msg)
        raise exception.DotHillConnectionError(message=msg)
    self._get_auth_token(xml.text.encode('utf8'))
    LOG.debug('session    key    =    %s', self._session_key)
    if (self._session_key is None):
        raise exception.DotHillAuthenticationError
