Ground work for hpa validation

Added the declarative schema for hpa validation. This is the ground
layering work for hpa validation.

Change-Id: I2ff30489e191ecf187711e1025f6e3faee9946c3
Issue-ID: VNFSDK-194
Signed-off-by: Lianhao Lu <lianhao.lu@intel.com>
diff --git a/requirements.txt b/requirements.txt
index 45562d5..e8df081 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,4 @@
-ruamel.yaml<0.16
+ruamel.yaml<0.15
 requests>=2.3.0
 #apache-ariatosca==0.1.1
 stevedore >= 1.9.0