Add a "simple track" mode

[danielmitterdorfer]

At the moment, tracks always need to reside in Rally's directory tree and it also needs to be managed by git. However, sometimes a user may just wants to run a very simple one-off benchmark and then all this complexity is not needed.

For these cases we could allow the user to provide an arbitrary directory on the machine where a custom track resides. It can also contain (optional) data files and Python extensions (custom runners and parameter sources). However, it does not need to be managed by git.

In the future, this simple mode will have one drawback though: Once we distribute the load test driver (see #257), the simple mode can only be used if the user does want to run a distributed benchmark.