Add priorityClassName support

[pracucci]

We would like to add a priorityClassName to etcd pods created by the operator, but it looks unsupported. We could contribute it but, before doing it, I'm wondering what's the sentiment. Is it a feature you may be willing to accept?

[nberlee]

@pracucci Because I also needed it, I have implemented it on my fork which has a working release. I would have preferred if cbws was continuing his work, but that does not seem the case.

see https://github.com/on2itsecurity/etcd-operator