Clean imports in code
Clean imports in code

In some part in the code we import objects. In the Openstack style guidelines
they recommend to import only modules. [1]: "Do not import objects, only modules".

[1] https://docs.openstack.org/hacking/0.10.3/

Change-Id: I646dd8a926047d63c12aa0802ce80977817b503b

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow