Use try-except block for parameters for heat stack

This commit adds a try-except block when populating parameters for
creating the heat stack. For example, if a user forgets to create an
external network, we throw an exception in
shaker.openstack.clients.neutron.choose_external_net, however we continue
even though connection to openstack would fail in server.py to accomodate
for scenarios that do not need OpenStack connection. We need to fail and
exit before creating heat stack if one of the required parameters is missing.

Change-Id: Ic5a10a83f51dc7f0c17d31eeca5a94ade8ee1409

shaker/engine/deploy.py

@@ -233,11 +233,13 @@ class Deployment(object):
 
         self.flavor_name = flavor_name
         self.image_name = image_name
+        self.stack_name = 'shaker_%s' % utils.random_string()
+        self.dns_nameservers = dns_nameservers
+        # intiailizing self.external_net last so that other attributes don't
+        # remain uninitialized in case user forgets to create external network
         self.external_net = (external_net or
                              neutron.choose_external_net(
                                  self.openstack_client.neutron))
-        self.stack_name = 'shaker_%s' % utils.random_string()
-        self.dns_nameservers = dns_nameservers
 
     def _get_compute_nodes(self, accommodation):
         try:
@@ -277,13 +279,18 @@ class Deployment(object):
         LOG.debug('Rendered template: %s', rendered_template)
 
         # create stack by Heat
-        merged_parameters = {
-            'server_endpoint': server_endpoint,
-            'external_net': self.external_net,
-            'image': self.image_name,
-            'flavor': self.flavor_name,
-            'dns_nameservers': self.dns_nameservers,
-        }
+        try:
+            merged_parameters = {
+                'server_endpoint': server_endpoint,
+                'external_net': self.external_net,
+                'image': self.image_name,
+                'flavor': self.flavor_name,
+                'dns_nameservers': self.dns_nameservers,
+            }
+        except AttributeError as e:
+            LOG.error('Failed to gather required parameters to create '
+                      'heat stack: %s', e)
+            exit(1)
         merged_parameters.update(specification.get('template_parameters', {}))
 
         self.has_stack = True