Don't use swift roles when no swift

If openstack doesn't have Swift installation this role assign is
unnecessary and cause bugs and fails for other tests.

Change-Id: If3bbc49cde6144e883daaf77b8f551a0562c84c8
diff --git a/tempest/common/isolated_creds.py b/tempest/common/isolated_creds.py
index 8166de5..2613911 100644
--- a/tempest/common/isolated_creds.py
+++ b/tempest/common/isolated_creds.py
@@ -160,9 +160,11 @@
         email = data_utils.rand_name(root) + suffix + "@example.com"
         user = self._create_user(username, self.password,
                                  tenant, email)
-        # NOTE(andrey-mp): user needs this role to create containers in swift
-        swift_operator_role = CONF.object_storage.operator_role
-        self._assign_user_role(tenant, user, swift_operator_role)
+        if CONF.service_available.swift:
+            # NOTE(andrey-mp): user needs this role to create containers
+            # in swift
+            swift_operator_role = CONF.object_storage.operator_role
+            self._assign_user_role(tenant, user, swift_operator_role)
         if admin:
             self._assign_user_role(tenant, user, CONF.identity.admin_role)
         return self._get_credentials(user, tenant)