Use ValidateV2 before Gitaly starts
The usage of ValidateV2() is guarded behind an envvar to allow proper
testing and config verification in staging and production before
enabling it by default.
Some functions invoked by Validate() was setting default values beside
actual validation, so this logic was moved to SetDefaults() to bring
both validation functions more closer to each other.
I've marked Validate() as deprecated and now the tests use
ValidateV2() instead.
Related to #5028
Edited by Ahmad Sherif