Add minimal_instance_type config option

for integration tests. Use a flavor similar to m1.micro for that.
It is enough to start both minimal and functional tests images with no
serious payload and should decrease the amount of RAM consumed by VMs
in corresponding tests four-fold.

Change-Id: I378b5a3cde020176ac45ebf8d05bd13278861a4e
Related-Bug: #1435285
diff --git a/scenario/scenario_base.py b/scenario/scenario_base.py
index 77c3624..dd25b89 100644
--- a/scenario/scenario_base.py
+++ b/scenario/scenario_base.py
@@ -31,6 +31,11 @@
         if not self.conf.instance_type:
             raise self.skipException("No flavor configured to test")
 
+        if not self.conf.minimal_image_ref:
+            raise self.skipException("No minimal image configured to test")
+        if not self.conf.minimal_instance_type:
+            raise self.skipException("No minimal flavor configured to test")
+
     def launch_stack(self, template_name, expected_status='CREATE_COMPLETE',
                      parameters=None, **kwargs):
         template = self._load_template(__file__, template_name, self.sub_dir)