core: support non-unicode output of programs

Some commands can output characters that are not decodable by
codecs.utf_8_decode. One example is nmcli (used by ovirt-host-deploy
plugin, that has a bug (reported and confirmed) retrieving the
vendor string with certain realteks and prints to stdout
non-utf-8-decodable strings.

Change-Id: I121d975083f02957aad927305d2cd9157c77b3ea
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>