Add support for following logs

[hongbin]

In Docker, it is possible to use the '--follow' option to follow the logs of a container.

  $ docker logs --help

  Usage: docker logs [OPTIONS] CONTAINER

  Fetch the logs of a container

  Options:
        ...
    -f, --follow Follow log output
        ...

This feature is useful. Zun is better to support this options as well. Note that the '--follow' requires streaming output to API users. The implementation might be similar as how we support interactive execute: https://review.openstack.org/#/c/445234/16

[1] https://github.com/openstack/zun/blob/master/zun/websocket/websocketproxy.py
[2] https://review.openstack.org/#/c/445234/16

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/571952