Problem/Motivation
Over in #2202511: Implement migration groups we are adding group support and there was talk of adding a way to store configuration for the group.
Storage of configuration shared among related migrations, most commonly source information - migrate_d2d stores database connection info in the group, wordpress_migrate stores the location of the source WXR file, etc.
Proposed resolution
Discuss our options.
Remaining tasks
Write the patch.
User interface changes
n/a
API changes
Possible additions.