[image-build] Take install tree from another variant

Add a configuration option to use install tree from another variant.

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

doc/configuration.rst

@@ -613,7 +613,10 @@ Image Build Settings
     automatically transformed into format suitable for ``koji``. A repo for the
     currently built variant will be added as well.
 
-    Please don't set ``install_tree`` as it would get overriden by pungi.
+    Please don't set ``install_tree``. This gets automatically set by *pungi*
+    based on current variant. You can use ``install_tree_from`` key to use
+    install tree from another variant.
+
     The ``format`` attr is [('image_type', 'image_suffix'), ...].
     See productmd documentation for list of supported types and suffixes.
 
@@ -655,6 +658,9 @@ Example
 
                 # only build this type of image on x86_64
                 'arches': ['x86_64']
+
+                # Use install tree from Everything variant.
+                'install_tree_from': 'Everything',
             }
         ]
     }