The pypi package 'docker-py' [1] has been renamed to 'docker' [2].
It is better to move to the new 'docker' package because the old
package will be deprecated and all the new features will go into
the new package only.

Package 'docker' has been added to requirements [3]. The old
package 'docker-py' is still allowed to be in the global requirements
during the transition period but it should be removed after all or
most of the projects finsih the migration.

[1] https://pypi.python.org/pypi/docker-py
[2] https://pypi.python.org/pypi/docker
[3] https://review.openstack.org/#/c/423715/

Change-Id: Ibcd5a57a1fbf55dcc5a690e41f20917f95b63da0