update architecture documentation

[gordon chung]

we've changed design to have processing all at notification agent. also we've split alarming.

we need update docs

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/222674

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/222674
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=15ea3dfaa5a284e8ecad8383fd8a459daee6ba63
Submitter: Jenkins
Branch: master

commit 15ea3dfaa5a284e8ecad8383fd8a459daee6ba63
Author: gordon chung <email address hidden>
Date: Fri Sep 11 11:28:00 2015 -0400

    update architecture documentation

    - note that pipeline management is only done at notification agent
    - remove references to rpc publishing
    - note that Aodh exists
    - update diagrams to show above.
    - add note regarding ability to stream samples from polling

    Change-Id: If1a483b84b3f20409b2a1c893ff17eda408e55f2
    Closes-Bug: #1494767