Don't always set imagePullPolicy to IfNotPresent

[consideRatio]

I think this should be configured to not default to explicitly setting IfNotPresent but instead omitting it by default. The default behavior when omitted is as Always if the image tag is latest, and otherwise IfNotPresent. This fix would be similar to jupyterhub/kubespawner#807 and shouldn't be a change that is disruptive for users, but rather a possible bugfix.

    image_pull_policy = Unicode(
        "IfNotPresent",
        help="The image pull policy of the docker image specified in ``image``",
        config=True,
    )

[consideRatio]

Closed by #770