doc: restructure the image building documentation

Main goal: consolidate the information about image
building under the same documentation page, and move
plugin-specific details inside plugin pages.
No plugin-specific information should live outside
those pages.

More details:
- move the detailed documentation about sahara-image-pack
  from the contributor guide to the new dedicated page
  in the user manual;
- remove the vanilla and cdh pages which describes building
  images with sahara-image-create, and move the common
  information to new sahara-image-create page
  in the user manual;
- add the matrix of supported plugin versions and
  supported building technology for each plugin inside
  the respective <plugin>-plugin.rst;
- add the redirects for the removed pages (only for master
  and rocky, where this change should be backported).
- remove few details not really needed (e.g. how to convert
  to VMDK images, location of cloud-init packages, etc,
  which do not really belong here).

Change-Id: I8398a7ad625276d8f11d743688ba71902a7e1adc

doc/source/user/spark-plugin.rst

@@ -10,14 +10,34 @@ support.
 Images
 ------
 
-For cluster provisioning, prepared images should be used. The Spark plugin
-has been developed and tested with the images generated by
-sahara-image-elements:
+For cluster provisioning, prepared images should be used.
 
-* https://git.openstack.org/cgit/openstack/sahara-image-elements
+.. list-table:: Support matrix for the `spark` plugin
+   :widths: 15 15 20 15 35
+   :header-rows: 1
 
-The latest Ubuntu images generated by sahara-image-elements have Cloudera
-CDH 5.4.0 HDFS and Apache Spark installed.
+   * - Version
+       (image tag)
+     - Distribution
+     - Build method
+     - Version
+       (build parameter)
+     - Notes
+
+   * - 2.3
+     - Ubuntu 16.04
+     - sahara-image-create
+     - 2.3.0
+     - based on CDH 5.11
+
+   * - 2.2
+     - Ubuntu 16.04
+     - sahara-image-create
+     - 2.2.0
+     - based on CDH 5.11
+
+For more information about building image, refer to
+:doc:`building-guest-images`.
 
 The Spark plugin requires an image to be tagged in the sahara image registry
 with two tags: 'spark' and '<Spark version>' (e.g. '1.6.0').
@@ -31,6 +51,15 @@ slaves), stop all services and so on. As such (and as opposed to CDH HDFS
 daemons), Spark is not deployed as a standard Ubuntu service and if the
 virtual machines are rebooted, Spark will not be restarted.
 
+Build settings
+~~~~~~~~~~~~~~
+
+When ``sahara-image-create`` is used, you can override few settings
+by exporting the corresponding environment variables
+before starting the build command:
+
+* ``SPARK_DOWNLOAD_URL`` - download link for Spark
+
 Spark configuration
 -------------------