defaults: add support for token_file env

lib/vault/defaults.rb

@@ -53,18 +53,16 @@ def address
       # The vault token to use for authentiation.
       # @return [String, nil]
       def token
-        if !ENV["VAULT_TOKEN"].nil?
-          return ENV["VAULT_TOKEN"]
-        end
+        ENV["VAULT_TOKEN"] || fetch_from_disk("VAULT_TOKEN_FILE", VAULT_DISK_TOKEN)
+      end
 
-        if VAULT_DISK_TOKEN.exist? && VAULT_DISK_TOKEN.readable?
-          return VAULT_DISK_TOKEN.read.chomp
+      def fetch_from_disk(env_var, default_path)
+        path = ENV[env_var] ? Pathname.new(ENV[env_var]) : default_path
+        if path&.exist? && path.readable?
+          return path.read.chomp
         end
-
-        nil
       end
 
-
       # Vault Namespace, if any.
       # @return [String, nil]
       def namespace