MXS-862: Implement GSSAPI backend authentication

The GSSAPI backend authentication is based on tokens.  The server first
sends the service principal name which is used for token generation. The
client then retrieves a token from the GSSAPI server which it sends to the
backend server. If the server can verify the authenticity of the token,
authentication is successful.

This module can be used with both GSSAPIAuth and MySQLAuth modules.

server/modules/protocol/MySQL/mysql_common.c

@@ -1354,7 +1354,7 @@ mxs_auth_state_t gw_send_backend_auth(DCB *dcb)
      */
     const char* auth_plugin_name =  DEFAULT_MYSQL_AUTH_PLUGIN;
 
-    long bytes = response_length(conn, local_session.user, local_session.client_sha1,
+    long bytes = response_length(conn, local_session.user, curr_passwd,
                                  local_session.db, auth_plugin_name);
 
     // allocating the GWBUF