netinfo.show() : Sort interface state.

The `tests/integration/vlan_test.py` will randomly fail
regarding the ordering of interface state. It's caused by
unsorted interface state list before comparing in
`assertlib.assert_state()`. The fix would be sort interface state in
`netinfo.show()`.

Signed-off-by: Gris Ge <fge@redhat.com>

libnmstate/netinfo.py

@@ -1,5 +1,5 @@
 #
-# Copyright 2018 Red Hat, Inc.
+# Copyright 2018-2019 Red Hat, Inc.
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -15,6 +15,7 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #
 from __future__ import absolute_import
+from operator import itemgetter
 
 from libnmstate import nm
 from libnmstate import validator
@@ -78,6 +79,8 @@ def interfaces():
 
         info.append(iface_info)
 
+    info.sort(key=itemgetter('name'))
+
     return info