Externalize car configuration

[danielmitterdorfer]

For details see #195.

Sub-tasks:

• Define a car repository format (probably very similar to More flexible configuration of benchmark candidates #195)
• Read and merge all configuration files
• Move some responsibilities from the launcher to the provisioner (e.g definition of node names)

As a simplification we will (temporarily) disallow to deploy multiple nodes on one host.

[danielmitterdorfer]

I've added the breaking label as we will remove the car two_nodes temporarily (until we've implemented #71).