Move well-known env variable names into a common place

[johscheuer]

What would you like to be added/changed?

In https://github.com/FoundationDB/fdb-kubernetes-operator/pull/2028/files#diff-ac49e64b9090afcbc491d5b0591784eb20d910e031838c8a4ddcac9593a4d0d9 we added a new common place to add well-known env variable names. We should move all env variables into this common place and only reference the constant.