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