iotests.py: don't abort if IMGKEYSECRET is undefined

Instead of using os.environ[], use .get with a default of empty string
to match the setup in check to allow us to import the iotests module
(for debugging, say) without needing a crafted environment just to
import the module.
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20181221093529.23855-6-jsnow@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/qemu-iotests/iotests.py

@@ -63,7 +63,7 @@ socket_scm_helper = os.environ.get('SOCKET_SCM_HELPER', 'socket_scm_helper')
 debug = False
 
 luks_default_secret_object = 'secret,id=keysec0,data=' + \
-                             os.environ['IMGKEYSECRET']
+                             os.environ.get('IMGKEYSECRET', '')
 luks_default_key_secret_opt = 'key-secret=keysec0'