[trivial] adds missing ssl_ca config option

config.yaml

@@ -79,6 +79,12 @@ options:
     default:
     description: |
       Base64 encoded SSL key to use with certificate specified as ssl_cert.
+  ssl_ca:
+    type: string
+    default:
+    description: |
+      Base64 encoded SSL CA to use with the certificate and key provided - only
+      required if you are providing a privately signed ssl_cert and ssl_key.
   # General Swift Proxy configuration
   bind-port:
     default: 8080