Previously, when running one of the following commands:

  kolla-ansible deploy --check
  kolla-ansible genconfig --check

deployment or configuration generation fails for various reasons.

MariaDB fails to lookup the existing cluster.

Keystone fails to generate cron config.

Nova-cell fails to get the cell settings.

Closes-Bug: #2002661
Change-Id: I5e765f498ae86d213d0a4379ca5d473db1499962

Deploy a small cloud. Add one host to the compute group in the
inventory, and scale out:

$ kolla-ansible deploy --limit <new compute host>

The command succeeds, but creating an instance fails with the following:

    Host 'compute0' is not mapped to any cell

This happens because we only discover computes on the first host in the
cell's nova conductor group. If that host is not in the specified limit,
the discovery will not happen.

This change fixes the issue by running compute discovery when any ironic
or virtualised compute hosts are in the play batch, and delegating it to
a conductor.

Change-Id: Ie984806240d147add825ffa8446ae6ff55ca4814
Closes-Bug: #1869371

This patch adds initial support for deploying multiple Nova cells.

Splitting a nova-cell role out from the Nova role allows a more granular
approach to deploying and configuring Nova services.

A new enable_cells flag has been added that enables the support of
multiple cells via the introduction of a super conductor in addition to
cell-specific conductors. When this flag is not set (the default), nova
is configured in the same manner as before - with a single conductor.

The nova role now deploys the global services:

* nova-api
* nova-scheduler
* nova-super-conductor (if enable_cells is true)

The nova-cell role handles services specific to a cell:

* nova-compute
* nova-compute-ironic
* nova-conductor
* nova-libvirt
* nova-novncproxy
* nova-serialproxy
* nova-spicehtml5proxy
* nova-ssh

This patch does not support using a single cell controller for managing
more than one cell. Support for sharing a cell controller will be added
in a future patch.

This patch should be backwards compatible and is tested by existing CI
jobs. A new CI job has been added that tests a multi-cell environment.

ceph-mon has been removed from the play hosts list as it is not
necessary - delegate_to does not require the host to be in the play.

Documentation will be added in a separate patch.

Partially Implements: blueprint support-nova-cells
Co-Authored-By: Mark Goddard <mark@stackhpc.com>
Change-Id: I810aad7d49db3f5a7fd9a2f0f746fd912fe03917