Add scoring engines to database and API layers

A Scoring Module needs to expose a list of available
scoring engines through API and Watcher CLI. This list
is stored in database and synchronized by Decision Engine.

Partially-Implements: blueprint scoring-module
Change-Id: I32168adeaf34fd12a731204c5b58fe68434ad087
APIImpact

watcher/common/exception.py

@@ -274,6 +274,14 @@ class EfficacyIndicatorAlreadyExists(Conflict):
     msg_fmt = _("An action with UUID %(uuid)s already exists")
 
 
+class ScoringEngineAlreadyExists(Conflict):
+    msg_fmt = _("A scoring engine with UUID %(uuid)s already exists")
+
+
+class ScoringEngineNotFound(ResourceNotFound):
+    msg_fmt = _("ScoringEngine %(scoring_engine)s could not be found")
+
+
 class HTTPNotFound(ResourceNotFound):
     pass