Replace the usage of 'os' with 'os_primary'

Read-only property 'os' has moved to 'os_primary' in version 'Pike'
and will be removed in version 'Queens', so this is to replace
the usage of 'os' with 'os_primary'.

Change-Id: I9f7c13da05a8c4a63529c11aa6213a7269abee6d
diff --git a/tempest/api/compute/base.py b/tempest/api/compute/base.py
index 141b9f3..c9d6f02 100644
--- a/tempest/api/compute/base.py
+++ b/tempest/api/compute/base.py
@@ -204,7 +204,7 @@
             kwargs['name'] = data_utils.rand_name(cls.__name__ + "-server")
         tenant_network = cls.get_tenant_network()
         body, servers = compute.create_test_server(
-            cls.os,
+            cls.os_primary,
             validatable,
             validation_resources=cls.validation_resources,
             tenant_network=tenant_network,