add default toleration to build pods

[jhamman]

This implements a minimally invasive addition to the build pod spec, adding a default tolerations to build pods that match zero2jupyterhub.

hub.jupyter.org/dedicated=user:NoSchedule

cc @consideRatio

closes #851

[betatim]

LGTM and tests seem happy. Merging now, if this turns out to be controversial we can always revert it.

[jhamman]

Thanks @betatim - I'll push this to Pangeo's binder and report any problems.

[consideRatio]

Is a build pods task to build a single image? Then i think this is really good!

[consideRatio]

Hmmmm, one may consider using the custom user scheduler as well then btw, if not, they will stop scale downs unless there is 0 build pods for ten full minutes.

[jhamman]

Is a build pods task to build a single image? Then i think this is really good!

Yes, the build pod builds a single image. In my view, this is ideally done on the same node the notebook will be run, as it provides a higher likelihood that we can avoid a pull of the recently built image. I think there is still an element of chance here so some additional scheduling could help.

one may consider using the custom user scheduler as well then btw

This sounds like a great idea.