zdtm: fix decode_flav()

decode_flav() was doing 'if i in flavors:' where 'i' was an integer but
the keys from the flavors dict are strings 'h', 'ns' and 'uns'.

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>