Add API graduation/post-graduation requirements

Add some graduation requirements around APIS:

  1) REST API - we say only server projects go through incubation

  2) API stability - we want reasonable stability at graduation

  3) Python client library - seems to be an unstated requirement

and a post-graduation commitment to full API stability and backwards
compatibility.

Change-Id: Ifce4262fcec22a2cbe9880a984b46f3c082830b1
Add Heat integration post-graduation expectation

The idea here is that since Heat is part of the integrated release,
a release including a new project wouldn't feel fully integrated
if there was no support for that new project in Heat.

Change-Id: I84e7ff321c02ee42c2886429d55d4e13b58f8b59
Add Horizon integration post-graduation expectation

The idea here is that since Horizon is part of the integrated release,
a release including a new project wouldn't feel fully integrated
if there was no support for that new project in Horizon.

Change-Id: Ifb3319099cf2eaf8af9dfaf89708415eddf71334
Fix post-graduation expectation typo

Change-Id: Ic39ffeb75fe4470e05d28fcf7e0c3d491ad1f270
Merge "Add API graduation/post-graduation requirements"
Merge "Add Heat integration post-graduation expectation"
Merge "Add Horizon integration post-graduation expectation"
Merge "Fix post-graduation expectation typo"