implementing imgepullpolicy in zun

1) If an image is updated then, containers need to be recreated, in that case we need to repull the image if image is with the same tag.
2) If a user is performing some tests, he may not want to pull image again and again from repository. So we don't need to pull the image again and again , instead use it from local host.

So we need to have some policy for pulling the image, which can be specified during the container create.

It can have following values:
IfNotPresent: do not pull an image if it already exists on host.
Always: Always pull the image from repositery.

If no tag or latest tag is there for image then policy default will be always.