Must use the snap version of agents when using edge version of juju

unit_tests/test_zaza_charm_lifecycle_prepare.py

@@ -75,7 +75,6 @@ class TestCharmLifecyclePrepare(ut_utils.BaseTestCase):
         self.add_model.assert_called_once_with(
             'newmodel',
             config={
-                'agent-stream': 'proposed',
                 'default-series': 'xenial',
                 'image-stream': 'daily',
                 'test-mode': 'true',

zaza/charm_lifecycle/prepare.py

@@ -25,7 +25,6 @@ import zaza.model
 MODEL_DEFAULTS = {
     # Model defaults from charm-test-infra
     #   https://jujucharms.com/docs/2.1/models-config
-    'agent-stream': 'proposed',
     'default-series': 'xenial',
     'image-stream': 'daily',
     'test-mode': 'true',