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

requirements.txt

@@ -17,6 +17,7 @@ oslo.utils>=3.5.0 # Apache-2.0
 PasteDeploy>=1.5.0 # MIT
 pbr>=1.6 # Apache-2.0
 pecan>=1.0.0 # BSD
+PrettyTable>=0.7,<0.8 # BSD
 voluptuous>=0.8.6 # BSD License
 python-ceilometerclient>=2.2.1 # Apache-2.0
 python-cinderclient>=1.3.1 # Apache-2.0