Add support for Yum groups.

Groups are defined in /software/groups (better name appreciated), and
have flags to enable the mandatory, default and optional packages in
them.

Please note that this requires an upgrade of quattor/schema.

Documentation will follow in the web site.
Add explicit test for empty sets
Add a repoquery message to the list of Yum errors

This one may be triggered when expanding package groups.
Rearrange declaration of groups
Merge branch 'spma-groups'

Conflicts:
	ncm-spma/src/main/pan/components/spma/config-rpm.pan
	ncm-spma/src/main/perl/spma/yum.pm