Add assert:supports-standalone

This proposes the addition of a tag for documenting projects which
can be run standalone.

This idea was first mentioned by Jeremy Freudberg in
http://lists.openstack.org/pipermail/openstack-discuss/2020-April/013821.html

Change-Id: I5276d20027cc0ad2ff080290e932c58c248bd405

reference/tags/assert_supports-standalone.rst

@@ -0,0 +1,41 @@
+..
+  This work is licensed under a Creative Commons Attribution 3.0
+  Unported License.
+  http://creativecommons.org/licenses/by/3.0/legalcode
+
+.. _`tag-assert:supports-standalone`:
+
+==========================
+assert:supports-standalone
+==========================
+
+This tag is part of the assert category of tags, which are assertions
+made by the project team themselves about the maturity of their deliverables.
+
+The "assert:supports-standalone" tag asserts that the service can be
+operated without requiring other OpenStack services, if the operator
+so chooses.
+
+Application to current deliverables
+===================================
+
+.. tagged-projects:: supports-standalone
+
+
+Rationale
+=========
+
+While OpenStack services are designed to work well together to manage
+the datacenter as a whole, some services can be operated independently of
+each other for specific use cases and requirements. Operators need to know
+which services these are in order to consider their adoption.
+
+Requirements
+============
+
+* The core functionality of the service is not affected by being operated
+  without other OpenStack services and limitations are clearly documented.
+* The service either supports another form of authentication separate
+  from Keystone, or no authentication in the case of a single tenant
+  environment.
+* The project tests and gates this deployment strategy.

reference/tags/index.rst

@@ -55,6 +55,7 @@ project doesn't actually meet the documented criteria.
    assert_supports-upgrade
    assert_supports-accessible-upgrade
    assert_supports-rolling-upgrade
+   assert_supports-standalone
 
 Vulnerability Management Tags
 =============================