[live-media] Add live media phase

This phase builds live media in Koji using the Live Media Creator. It
runs in parallel with current live images, create ISO and image build
phases.

The documentation is updated to explain how to configure this.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

doc/configuration.rst

@@ -135,6 +135,7 @@ Options
      * iso
      * live
      * image-build
+     * live-media
 
        .. note::
 
@@ -646,6 +647,30 @@ Live Images Settings
       * ``scratch`` (*bool*) -- only RPM-wrapped images can use scratch builds,
         but by default this is turned off
 
+Live Media Settings
+===================
+
+**live_media**
+    (*dict*) -- configuration for ``koji spin-livemedia``; format:
+    ``{variant_uid_regex: [{opt:value}]}``
+
+    Available options:
+
+      * ``target`` (*str*)
+      * ``arches`` (*[str]*) -- what architectures to build the media for; by default uses
+        all arches for the variant.
+      * ``kickstart`` (*str*) -- name of the kickstart file
+      * ``ksurl`` (*str*)
+      * ``ksversion`` (*str*)
+      * ``scratch`` (*bool*)
+      * ``release`` (*str*) -- a string with the release, or explicit ``None``
+        for using compose date and respin.
+      * ``skip_tag`` (*bool*)
+      * ``name`` (*str*)
+      * ``repo`` (*[str]*) -- external repo
+      * ``repo_from`` (*[str]*) -- list of variants to take extra repos from
+      * ``title`` (*str*)
+
 
 Image Build Settings
 ====================