make the tie to node/group GUI similar for

• free-stype project: "Tie this project to a node"
• matrix project, tie-matrix-parent plugin: "Tie parent build to a node"
• matrix project, "Build on multiple nodes"

While the first two GUI items provide a plain list with nodes and groups, the last GUI item has a tree structure that sperated nodes from groups. From our perspective, this is preferred, asn it get easier for a large number of nodes and groups.

See also JENKINS-7016 for an explanation form kbertelson, why we did the plugin this way. Again, from our perspective naming nodes and groups with a prefix should not be the solution, as this is more a hacky work around. We are aware that "Build on multiple nodes" will become more complex than the other two GUI items, as future version may also take into account build specific configurations on specific nodes/groups. However, giving the user a similar approach for all three GUI item will make things easier to unserstand.