Adding introspection for package groups (in yum)

Produces output like:

    # salt-call pkg.group_detail 'SNMP Support'
    conditional packages:
        installed:
        not installed:
    default packages:
        installed:
            - net-snmp-utils
        not installed:
    mandatory packages:
        installed:
            - net-snmp
        not installed:
    optional packages:
        installed:
            - net-snmp-python
        not installed:
            - net-snmp-perl