Add priorityClassName support

[pracucci]

We would like to add a priorityClassName to etcd pods created by the operator, but it looks unsupported to me. May you confirm it's unsupported and whether there's some interest into adding the support, please?

[Marlinc]

Hey there,

This appears to be unsupported at the moment, although I do think it does make sense to add it to improve the survivability of etcd clusters. I'm interested in adding support for this as this would also help us. I might create a pull request for this on our fork as this repository doesn't appear to be maintained anymore.

Could you maybe also open the same issue on https://github.com/cbws/etcd-operator?