cfn-hup triggers post.update events every time it's called

[Steve Baker]

From https://review.openstack.org/#/c/21864/

> cfn-hup currently triggers the post.update events every time it's called -- even when the metadata didn't change.

Need to fix this so we only trigger updates when the metadata has changed

[Simon Pasquier (simon-pasquier)]

As I understand, the issue appears when there is more than one resource monitored by cfn-hup hooks.
The cfn-hup workflow is something like this:
- process hook #1
 - retrieve metadata of resource #1
 - compare with /var/cache/heat-cfntools/last_metadata
 - run hook
 - store metadata of resource #1 in /var/cache/heat-cfntools/last_metadata
- process hook #2
 - retrieve metadata of resource #2
 - compare with /var/cache/heat-cfntools/last_metadata <<< different since it contains metadata of resource #1
 - run hook
 - store metadata of resource #2 in /var/cache/heat-cfntools/last_metadata

[Simon Pasquier (simon-pasquier)]

Additionally bug #1205375 describes another reason why cfn-hup would execute a hook at every run.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/96401

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/96401
Committed: https://git.openstack.org/cgit/openstack/heat-cfntools/commit/?id=5f7025558cf468792d1c18605667888a5d775b36
Submitter: Jenkins
Branch: master

commit 5f7025558cf468792d1c18605667888a5d775b36
Author: Ethan Lynn <email address hidden>
Date: Thu May 29 14:49:14 2014 +0800

    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