Doc dev-quickstart should also be changed after converting to new Devstack plugin model

[Lan Qi song]

After this patch https://review.openstack.org/#/c/168194/ has been merged , we can use an easier way to enable magnum in devstack, but the dev-quickstart document still indicated the old way to integrate magnum in devstack, we need update it.

Also, after we repeat to do "stack" and "unstack", we will find following contents are repeated at the end of global-requirements.txt :

python-kubernetes>=0.2
docker-py>=0.5.1
python-kubernetes>=0.2
docker-py>=0.5.1

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/171529
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=41aa2c92ade9bb52fbfaca04790bab2e924b76f8
Submitter: Jenkins
Branch: master

commit 41aa2c92ade9bb52fbfaca04790bab2e924b76f8
Author: lqslan <email address hidden>
Date: Wed Apr 8 17:03:56 2015 +0800

    Update dev-quickstart doc to match new devstack model

    Magnum has been converted to new devstack plugin model, so we need
    to update the dev-quickstart doc.

    This patch also fix a content repeat issue in devstack plugin script.

    Change-Id: Ibcb93a7e83a5c7f3a3f3727b957b1c45beeed5ae
    Closes-Bug: #1441514