urwid: Force utf8 encoding

The UI got into trouble, once the encoding of the console got changed.
Now the UI is always usnig UTF-8.

Change-Id: Ieccfe87e8bd11ac55453f3d01a62cde9310a9be2
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=980371
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=980452
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>