'Persist' __getattr__ results

This patch adds a new decorator that helps creating 'cachable'
__getattr__ methods. The decorator sets the returned value as an
attribute of the instance.

The patch also decorates `Pipeline`'s __getattr__ method

Implements blueprint storage-pipeline

Change-Id: I6b052d536d5c6e6edaeec884b3c4a0e8703b1036