Added Goal model into Watcher DB

In this changeset, I added the Goal model into Watcher.
This implies a change into the Watcher DB schema

Partially Implements: blueprint get-goal-from-strategy

Change-Id: I5b5b0ffc7cff8affb59f17743e1af0e1277c2878

watcher/common/exception.py

@@ -166,6 +166,14 @@ class InvalidUuidOrName(Invalid):
     msg_fmt = _("Expected a logical name or uuid but received %(name)s")
 
 
+class GoalNotFound(ResourceNotFound):
+    msg_fmt = _("Goal %(goal)s could not be found")
+
+
+class GoalAlreadyExists(Conflict):
+    msg_fmt = _("A goal with UUID %(uuid)s already exists")
+
+
 class AuditTemplateNotFound(ResourceNotFound):
     msg_fmt = _("AuditTemplate %(audit_template)s could not be found")