python/samba/tests: Port samba.tests.blackbox.samba_dnsupdate for PY2/PY3

python/samba/tests: Port samba.tests.blackbox.samba_dnsupdate for PY2/PY3

Make sure either the output of tests and/or the item we are searching match
in type. Output of cmd in python3 is bytes, depending on the was the test is
written it may be easier just to convert all output or just a single string
that is used in the test

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>