Add Overload standard deviation strategy

The main purpose of this strategy is to choose the pair VM:dest_host that
minimizes the standard deviation in a cluster best.

Change-Id: I95a31b7bcab83411ef6b6e1e01818ca21ef96883
Implements: blueprint watcher-overload-sd

watcher/common/exception.py

@@ -298,6 +298,14 @@ class NoAvailableStrategyForGoal(WatcherException):
     msg_fmt = _("No strategy could be found to achieve the '%(goal)s' goal.")
 
 
+class NoMetricValuesForVM(WatcherException):
+    msg_fmt = _("No values returned by %(resource_id)s for %(metric_name)s.")
+
+
+class NoSuchMetricForHost(WatcherException):
+    msg_fmt = _("No %(metric)s metric for %(host)s found.")
+
+
 # Model
 
 class InstanceNotFound(WatcherException):