Add res_last_path to store last metadata of a resource

Add res_last_path='/var/cache/heat-cfntools/last_metadata_<resource>'
in Metadata.retrieve() function to store metadata of a resource.
Remove md5 check between current metadata and last metadata, json.load()
will organize json structure.

Change-Id: Ie0c31a748f0add3fcab6a579431a28b60051f601
Closes-Bug: #1205375
Partial-Bug: #1133049
Merge "Add res_last_path to store last metadata of a resource"