Allow customizing image name and volume id

The computation of image name has been moved to a separate function.
This allowed simplification in how work dir for isos is computed, but
the result is not changed.

The live image phase has some special casing for names of RPM wrapped
ISOs. This is moved to the actual phase. Since this is (and has been)
undocumented, there should not be many users of this special case.

The documentation is updated to describe how image names and volume ids
are determined and how the process can be customized.

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

doc/configuration.rst

@@ -127,7 +127,6 @@ Options
 **variants_file** [mandatory]
     (*scm_dict* or *str*) -- reference to variants XML file that defines release variants and architectures
 
-
 Example
 -------
 ::
@@ -147,6 +146,80 @@ Example
     }
 
 
+Image Naming
+============
+
+Both image name and volume id are generated based on the configuration. Since
+the volume id is limited to 32 characters, there are more settings available.
+The process for generating volume id is to get a list of possible formats and
+try them sequentially until one fits in the length limit. If substitutions are
+configured, each attempted volume id will be modified by it.
+
+For layered products, the candidate formats are first
+``image_volid_layered_product_formats`` followed by ``image_volid_formats``.
+Otherwise, only ``image_volid_formats`` are tried.
+
+If no format matches the length limit, an error will be reported and compose
+aborted.
+
+Options
+-------
+
+**image_name_format** [optional]
+    (*str*) -- Python's format string to serve as template for image names
+
+    This format will be used for all phases generating images. Currently that
+    means ``createiso``, ``live_images`` and ``buildinstall``.
+
+    Available keys are:
+     * compose_id
+     * variant
+     * arch
+     * disc_type
+     * disc_num
+     * suffix
+     * release_short
+     * version
+
+**image_volid_formats** [optional]
+    (*list*) -- A list of format strings for generating volume id.
+
+    The available keys are:
+     * compose_id
+     * variant
+     * arch
+     * disc_type
+     * release_short
+     * version
+     * base_product_short
+     * base_product_version
+
+**image_volid_layered_product_formats** [optional]
+    (*list*) -- A listof format strings for generating volume id for layered
+    products. The keys available are the same as for ``image_volid_formats``.
+
+**volume_id_substitutions** [optional]
+    (*dict*) -- A mapping of string replacements to shorten the volume id.
+
+Example
+-------
+::
+
+    # Image name respecting Fedora's image naming policy
+    image_name_format = "%(release_short)s-%(variant)s-%(disc_type)s-%(arch)s-%(version)s%(suffix)s"
+    # Use the same format for volume id
+    image_volid_formats = [
+        "%(release_short)s-%(variant)s-%(disc_type)s-%(arch)s-%(version)s"
+    ]
+    # No special handling for layered products, use same format as for regular images
+    image_volid_layered_product_formats = []
+    # Replace "Cloud" with "C" in volume id etc.
+    volume_id_substitutions = {
+        'Cloud': 'C',
+        'Alpha': 'A',
+        'Beta': 'B',
+        'TC': 'T',
+    }
 
 
 Createrepo Settings