qpid: plain auth needs cyrus-sasl-plain package

On fedora 21, qpidd cannot authenticate user, because
cyrus-sasl-plain is no more automatically installed.

This change fixes that.

Change-Id: I74452f40723881291b8c7577e5509da1c0e4e6e5
diff --git a/lib/rpc_backend b/lib/rpc_backend
index 981b80b..30d300b 100644
--- a/lib/rpc_backend
+++ b/lib/rpc_backend
@@ -342,6 +342,7 @@
             install_package sasl2-bin
         elif is_fedora; then
             install_package cyrus-sasl-lib
+            install_package cyrus-sasl-plain
         fi
         local sasl_conf_file=/etc/sasl2/qpidd.conf
         sudo sed -i.bak '/PLAIN/!s/mech_list: /mech_list: PLAIN /' $sasl_conf_file