Added EfficacyIndicator model in DB

In this changeset, I created a new model named EfficacyIndicator
which is responsible for storing information regarding the efficacy
indicators that were computed by the strategy in its solution. Every
efficacy indicator should relate to a single Action Plan.

Partially Implements: blueprint efficacy-indicator

Change-Id: Ifc14ea5e16e92f032d7912c9b3fdc270af79cab6

watcher/common/exception.py

@@ -248,6 +248,14 @@ class ActionFilterCombinationProhibited(Invalid):
                 "prohibited")
 
 
+class EfficacyIndicatorNotFound(ResourceNotFound):
+    msg_fmt = _("Efficacy indicator %(efficacy_indicator)s could not be found")
+
+
+class EfficacyIndicatorAlreadyExists(Conflict):
+    msg_fmt = _("An action with UUID %(uuid)s already exists")
+
+
 class HTTPNotFound(ResourceNotFound):
     pass