Monthly snapshot for 2021.12.01

Add vault_harden_file_perms to set chmod 0550 on config/plugins path

The [Production
Hardening](https://learn.hashicorp.com/tutorials/vault/production-hardening)
have a bullet point "Allow minimal write privileges". It states: "its
executable binary or any Vault configuration files".

Prior to this change, the config and plugins path had chmod 0750, so
Vault could actually write config files and change plugins.

This commit adds a new parameter named vault_harden_file_perms (turned
off by default). When enabled, it changes the chmod of config and
plugins path to 0550 to effectively disallow Vault from writing into
these dirs.

Signed-off-by: Albin Kerouanton <albinker@gmail.com>