docs: move image formats doc to glance repo

[Brian Rosmaita]

The "Disk and container formats for images" section of the "VM Image Guide" in the openstack-manuals repo has gotten out date with respect to Glance image format changes. Additionally, it duplicates some information from documentation maintained by the Glance team.

To make the docs easier to maintain, we should make these changes:

(1) rewrite the Introduction page of the VM Image Guide so that it discusses the concept of "container format" and "disk format". Don't discuss the set of identifiers that Glance recognizes in the manual; instead, give a link to the appropriate place in the glance docs. Remove the two sub-pages (image-formats.rst and image-metadata.rst).

(2) merge the content from openstack-manuals/image-formats.rst and openstack-manuals/image-metadata.rst into docs/user/formats.rst in the glance repo

[Brian Rosmaita (brian-rosmaita)]

openstack-manuals change is https://review.opendev.org/#/c/678856/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/679692

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/679692
Committed: https://git.openstack.org/cgit/openstack/glance/commit/?id=c3e0b8436a2e2fdb53ab76bcc61fb61f06e73edf
Submitter: Zuul
Branch: master

commit c3e0b8436a2e2fdb53ab76bcc61fb61f06e73edf
Author: Brian Rosmaita <email address hidden>
Date: Mon Sep 2 21:14:02 2019 -0400

    Add disk format content from Image Guide

    The container_format and disk_format identifiers and their
    descriptions are being maintained in the Glance docs. Move some
    useful content on this topic over from the Image Guide before
    deleting it over there so it's only being maintained in one
    location.

    Change-Id: Ifa8de9d3a3a2aa78ee305aaebb23cd982b02dd91
    Partial-bug: #1842343

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/678856
Committed: https://git.openstack.org/cgit/openstack/openstack-manuals/commit/?id=d644cde37f1c6883f140c32f0228a7d7a7031485
Submitter: Zuul
Branch: master

commit d644cde37f1c6883f140c32f0228a7d7a7031485
Author: Brian Rosmaita <email address hidden>
Date: Tue Aug 27 10:12:19 2019 -0400

    Update the VM Image Guide

    Moves the content of two sub-pages into the Introduction page.
    The content that specified the legal values for the image properties
    content_format and disk_format has been consolidated into the
    documents that already describe that information that are maintained
    by the Glance team.

    Partial-bug: #1842343
    Change-Id: I8885d76b55ed67e9c22b49c86c7e2a9bdfcf03c6