[c-c list] add an optional argument to the list command (closes #2709703)
This optional argument allows to restrict listing to only
a given type of items (cubes, instances, configurations).
option_group_changed('port', 'main', 'web')
option_group_changed('query-log-file', 'main', 'web')
option_group_changed('profile', 'main', 'web')
option_group_changed('max-post-length', 'main', 'web')