Let track authors decide on the default challenge

[danielmitterdorfer]

For historic reasons every track needs to have a challenge "append-no-conflicts" to spare users to specify it on the command line with --challenge.

We should add a little bit more smarts:

• Add a new (optional) property for challenges in the track specification to select a challenge as default (and also verify that only one of them is true).
• If there is only one challenge (whatever its name), it is selected as the implicit default.

We will not implement this in a backwards compatible fashion but Rally will need to indicate very clearly to users how to fix their tracks.