docs: Rewrite host aggregate, availability zone docs

These closely related features are the source of a disproportionate
number of bugs and a large amount of confusion among users. The spread
of information around multiple docs probably doesn't help matters.

Do what we've already done for the metadata service and remote consoles
and clean these docs up. There are a number of important changes:

- All documentation related to host aggregates and availability zones is
  placed in one of three documents, '/user/availability-zones',
  '/admin/aggregates' and '/admin/availability-zones'. (note that there
  is no '/user/aggregates' document since this is not user-facing)

- References to these features are updated to point to the new location

- A glossary is added. Currently this only contains definitions for host
  aggregates and availability zones

- nova CLI commands are replaced with their openstack CLI counterparts

- Some gaps in related documentation are closed

Change-Id: If847b0085dbfb4c813d4a8d14d99346f8252bc19
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

doc/source/index.rst

@@ -152,7 +152,7 @@ Once you are running nova, the following information is extremely useful.
 * :doc:`Upgrades </user/upgrade>`: How nova is designed to be upgraded for minimal
   service impact, and the order you should do them in.
 * :doc:`Quotas </user/quotas>`: Managing project quotas in nova.
-* :doc:`Aggregates </user/aggregates>`: Aggregates are a useful way of grouping
+* :doc:`Aggregates </admin/aggregates>`: Aggregates are a useful way of grouping
   hosts together for scheduling purposes.
 * :doc:`Filter Scheduler </user/filter-scheduler>`: How the filter scheduler is
   configured, and how that will impact where compute instances land in your
@@ -225,6 +225,7 @@ looking parts of our architecture. These are collected below.
    contributor/ptl-guide
    reference/api-microversion-history.rst
    reference/conductor
+   reference/glossary
    reference/gmr
    reference/i18n
    reference/live-migration
@@ -242,8 +243,8 @@ looking parts of our architecture. These are collected below.
    reference/scheduler-hints-vs-flavor-extra-specs
    reference/isolate-aggregates
    user/index
-   user/aggregates
    user/architecture
+   user/availability-zones
    user/block-device-mapping
    user/cells
    user/cellsv2-layout