Make single source for KV values in enterprise helm

[Isan-Rivkin]

To configure the KV when using enterprise we must do it twice (i.e duplicate places to configure postgres).
There should be single source of truth that lakeFS/fluffy both use.

Regardless if it's using the Secrets, Env Vars, lakefsConfig/fluffyConfig - users should configure this once.

Another thing: add validation using .tpl that requires useDevPostgres: false if db config are set.

[Isan-Rivkin]

@talSofer check this issue seems very relevant to your current struggles :)