Use minimal image wherever possible

This changes most of the tests to use minimal image. There are
now two tests left that use fedora image.

[1] SoftwareConfigIntegrationTest.test_server_software_config
[2] CfnInitIntegrationTest.test_server_cfn_init

Out of which [1] is already disabled at the gate.

Change-Id: I309989de1e5584f3e345213c4b62893159365973
Related-Bug: #1628302
diff --git a/scenario/test_server_signal.py b/scenario/test_server_signal.py
index de42f02..4567e43 100644
--- a/scenario/test_server_signal.py
+++ b/scenario/test_server_signal.py
@@ -24,8 +24,8 @@
         """Check a server in a created network can signal to heat."""
         parameters = {
             'key_name': self.keypair_name,
-            'flavor': self.conf.instance_type,
-            'image': self.conf.image_ref,
+            'flavor': self.conf.minimal_instance_type,
+            'image': self.conf.minimal_image_ref,
             'timeout': self.conf.build_timeout,
         }