Mutable config

Talk about mutable config options, where they're supported and how to
reload them.

Change-Id: I1f1b02c6967b4b5e4bd52249e06006893115cb6c

doc/config-reference/source/config-overview.rst

@@ -7,6 +7,7 @@ OpenStack configuration overview
 
    common/conventions.rst
    config-format.rst
+   mutable.rst
 
 OpenStack is a collection of open source project components
 that enable setting up cloud services. Each component uses similar

doc/config-reference/source/mutable.rst

@@ -0,0 +1,28 @@
+==========================
+Changing config at runtime
+==========================
+
+OpenStack Newton introduces the ability to reload (or 'mutate') certain
+configuration options at runtime without a service restart. The following
+projects support this:
+
+* Compute (nova)
+
+Check individual options to discover if they are mutable.
+
+
+In practice
+~~~~~~~~~~~
+
+A common use case is to enable debug logging after a failure. Use the mutable
+config option called 'debug' (see :ref:`nova-logging`) to do this (providing
+``log_config_append`` has not been set). An admin user may perform the
+following steps:
+
+#. Log onto the compute node.
+#. Edit the config file (EG ``nova.conf``) and change 'debug' to ``True``.
+#. Send a SIGHUP signal to the nova process (For example, ``pkill -HUP nova``).
+
+A log message will be written out confirming that the option has been changed.
+If you use a CMS like Ansible, Chef, or Puppet, we recommend scripting these
+steps through your CMS.