Add metadata_encryption_key to glance-cache.conf.

Fixes LP Bug #1012752.

Change-Id: I802583e2766cd1533223638259e57e44b39448eb

etc/glance-cache.conf

@@ -133,3 +133,10 @@ s3_store_create_bucket_on_put = False
 # temporary buffer on disk. By default the platform's temporary directory
 # will be used. If required, an alternative directory can be specified here.
 # s3_store_object_buffer_dir = /path/to/dir
+
+# ================= Security Options ==========================
+
+# AES key for encrypting store 'location' metadata, including
+# -- if used -- Swift or S3 credentials
+# Should be set to a random string of length 16, 24 or 32 bytes
+# metadata_encryption_key = <16, 24 or 32 char registry metadata key>