Add script to orchestrate multiple composes

It may make sense to break a big compose into smaller chunks that can be
done independently. This script allows describing the smaller parts,
runs them with correct dependencies and arranges the result to look like
a single big compose.

All parts use the same koji event, that is either obtained from Koji, or
from command line argument.

JIRA: COMPOSE-2654
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

doc/multi_compose.rst

@@ -0,0 +1,60 @@
+.. _multi_compose:
+
+Managing compose from multiple parts
+====================================
+
+There may be cases where it makes sense to split a big compose into separate
+parts, but create a compose output that links all output into one familiar
+structure.
+
+The `pungi-orchestrate` tools allows that.
+
+It works with an INI-style configuration file. The ``[general]`` section
+contains information about identity of the main compose. Other sections define
+individual parts.
+
+The parts are scheduled to run in parallel, with the minimal amount of
+serialization. The final compose directory will contain hard-links to the
+files.
+
+
+General settings
+----------------
+
+**target**
+   Path to directory where the final compose should be created.
+**compose_type**
+   Type of compose to make.
+**release_name**
+   Name of the product for the final compose.
+**release_short**
+   Short name of the product for the final compose.
+**release_version**
+   Version of the product for the final compose.
+**release_type**
+   Type of the product for the final compose.
+**extra_args**
+   Additional arguments that wil be passed to the child Pungi processes.
+**koji_profile**
+   If specified, a current event will be retrieved from the Koji instance and
+   used for all parts.
+
+
+Partial compose settings
+------------------------
+
+Each part should have a separate section in the config file.
+
+It can specify these options:
+
+**config**
+   Path to configuration file that describes this part. If relative, it is
+   resolved relative to the file with parts configuration.
+**just_phase**, **skip_phase**
+   Customize which phases should run for this part.
+**depends_on**
+   A comma separated list of other parts that must be finished before this part
+   starts.
+**failable**
+   A boolean toggle to mark a part as failable. A failure in such part will
+   mark the final compose as incomplete, but still successful.