Create and populate htpasswd file if missing

If htpasswd authentication option is configured but the htpasswd file is
missing, populate it with a default user and automatically generated
password.
The password will be printed to stdout.

Signed-off-by: Liron Levin <liron@twistlock.com>

cmd/registry/config-example.yml

@@ -11,6 +11,10 @@ http:
   addr: :5000
   headers:
     X-Content-Type-Options: [nosniff]
+auth:
+  htpasswd:
+    realm: basic-realm
+    path: /etc/registry
 health:
   storagedriver:
     enabled: true