Add Voluptuous to validate the action parameters

We want a simplest way to validate the input parameters of an
Action through a schema.

APIImpact
DocImpact
Partially implements: blueprint watcher-add-actions-via-conf

Change-Id: I139775f467fe7778c7354b0cfacf796fc27ffcb2

watcher/common/exception.py

@@ -279,3 +279,7 @@ class HypervisorNotFound(WatcherException):
 
 class LoadingError(WatcherException):
     msg_fmt = _("Error loading plugin '%(name)s'")
+
+
+class ReservedWord(WatcherException):
+    msg_fmt = _("The identifier '%(name)s' is a reserved word")