Add imgfac submodule to meet ova dependencies

The ova creator script requires some recent imgfac features. Now the
latest imgfac code is added as a submodule to give the script access to
it.

Change-Id: I68d516e18783864e3010ab5b2da31a1d48836fd2
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>