New query to determine kickstartable channels

This PR comes with a new query to determine *kickstartable* channels by implementing a logic that was the outcome of a discussion on spacewalk-devel (see this [thread] (https://www.redhat.com/archives/spacewalk-devel/2013-March/msg00018.html)). A *kickstartable* channel is therefore a channel that:

1. has been used as the base for at least one kickstartable tree (distribution) that has an install type with a label matching either ``rhel*`` or ``fedora``
2. is available to the current user's org

Further there is now a need to distinguish between those *kickstartable channels* and channels that can be
used for creating kickstartable trees (distributions) which is a superset of the previous. I called those *kickstartable tree channels* and they are determined using the original query that has been syntactically rewritten as well.

Further this PR includes also fixes and improvements regarding unit tests involving kickstartable channels.

Thank you and feel free to discuss as always!