Doing a stack-update on every nested stack can take a long time for large stacks

[Steve Baker]

A 2 node tripleo stack creates ~140 nested stacks (!). Stack updates are sometimes done where no files or other environment changes are made, so there is a potential time saving in this case where only the nested stacks which really need updating should be updated.

I'll post profiling as comments as I try different approaches.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/273859

[OpenStack Infra (hudson-openstack)]

Change abandoned by Steve Baker (<email address hidden>) on branch: master
Review: https://review.openstack.org/273859
Reason: Not needed, implementation signature is going away https://review.openstack.org/#/c/272366

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/274921

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/273858
Committed: https://git.openstack.org/cgit/openstack/heat/commit/?id=95db9fb600583d4feed806da9a717855e010854b
Submitter: Jenkins
Branch: master

commit 95db9fb600583d4feed806da9a717855e010854b
Author: Steve Baker <email address hidden>
Date: Wed Feb 3 10:07:45 2016 +1300

    Refactor StackResource create/update stack args

    This is a pure refactoring change in preparation for using these
    arguments to determine whether the stack resource really needs to be
    updated.

    Change-Id: I27e73a82cd8021155ac42f34f22f0aad8fc8a48a
    Related-Bug: #1539362

[OpenStack Infra (hudson-openstack)]

Change abandoned by Steve Baker (<email address hidden>) on branch: master
Review: https://review.openstack.org/274921
Reason: ugh, _needs_update is used by convergence paths too. Plus what Zane said