Refactored Strategy selector to select from DB

In this changeset, I refactored the strategy selector to now
look into the Watcher DB instead of looking into the configuration
file.

Partially Implements: blueprint get-goal-from-strategy

Change-Id: I2bcb63542f6237f26796a3e5a781c8b62820cf6f

watcher/common/exception.py

@@ -287,7 +287,11 @@ class MetricCollectorNotDefined(WatcherException):
 
 
 class ClusterStateNotDefined(WatcherException):
-    msg_fmt = _("the cluster state is not defined")
+    msg_fmt = _("The cluster state is not defined")
+
+
+class NoAvailableStrategyForGoal(WatcherException):
+    msg_fmt = _("No strategy could be found to achieve the '%(goal)s' goal.")
 
 
 # Model