Add support for saving config

[jlongstaf]

Need to save BIG-IP configuration. Using the BIG-IP REST API, POST JSON {'command': 'save'} to /sys/config. Here's the complete implementation from LBaaS v1:

def save_config(self):
    """ Save the bigip configuration """
    request_url = self.bigip.icr_url + '/sys/config'
    payload = dict()
    payload['command'] = 'save'
    response = self.bigip.icr_session.post(
        request_url, data=json.dumps(payload),
        timeout=const.CONNECTION_TIMEOUT)
    if response.status_code < 400:
        return True
    else:
        Log.error('config', response.text)
        raise exceptions.BigIPClusterConfigSaveFailure(response.text)
    return False