Added purge script for soft deleted objects

This patchset implements the purge script as specified in its
related blueprint:

- The '--age-in-days' option allows to specify the number of
  days before expiry
- The '--max-number' option allows us to specify a limit on the number
  of objects to delete
- The '--audit-template' option allows you to only delete objects
  related to the specified audit template UUID or name
- The '--dry-run' option to go through the purge procedure without
  actually deleting anything
- The '--exclude-orphans' option which allows you to exclude from the
  purge any object that does not have a parent (i.e. and audit without
  a related audit template)

A prompt has been added to also propose to narrow down the number of
deletions to be below the specified limit.

Change-Id: I3ce83ab95277c109df67a6b5b920a878f6e59d3f
Implements: blueprint db-purge-engine

watcher/common/exception.py

@@ -283,3 +283,11 @@ class LoadingError(WatcherException):
 
 class ReservedWord(WatcherException):
     msg_fmt = _("The identifier '%(name)s' is a reserved word")
+
+
+class NotSoftDeletedStateError(WatcherException):
+    msg_fmt = _("The %(name)s resource %(id)s is not soft deleted")
+
+
+class NegativeLimitError(WatcherException):
+    msg_fmt = _("Limit should be positive")