Added Strategy model

In this changeset, I add the Strategy model as well as the DB
functionalities we need to manipulate strategies.

This changeset implies a DB schema update.

Partially Implements: blueprint get-goal-from-strategy

Change-Id: I438a8788844fbc514edfe1e9e3136f46ba5a82f2

watcher/common/exception.py

@@ -151,7 +151,7 @@ class InvalidIdentity(Invalid):
 
 
 class InvalidGoal(Invalid):
-    msg_fmt = _("Goal %(goal)s is not defined in Watcher configuration file")
+    msg_fmt = _("Goal %(goal)s is invalid")
 
 
 class InvalidUUID(Invalid):
@@ -174,6 +174,14 @@ class GoalAlreadyExists(Conflict):
     msg_fmt = _("A goal with UUID %(uuid)s already exists")
 
 
+class StrategyNotFound(ResourceNotFound):
+    msg_fmt = _("Strategy %(strategy)s could not be found")
+
+
+class StrategyAlreadyExists(Conflict):
+    msg_fmt = _("A strategy with UUID %(uuid)s already exists")
+
+
 class AuditTemplateNotFound(ResourceNotFound):
     msg_fmt = _("AuditTemplate %(audit_template)s could not be found")