Support multiple track repositories

[danielmitterdorfer]

With #69 we will introduce a separate Git repository for tracks. It would be great if Rally could handle multiple Git repositories for tracks. Not everybody can / want to share their benchmark data so users could create their own private repository for their benchmarks and can still use Rally.

Each track repository will get a unique name and a URL which will be stored in ~/.rally/rally.ini (we will add no tool support in the first step for such a narrow use case, just edit the file directly). The default repository (elastic) will be automatically created by Rally during the automatic config file migration process during the first startup.

Different track repositories will live under a repo-specific subfolder (e.g. $RALLY_HOME/tracks/default for the default track repository).

We will not implement a fancy fallback logic to find the correct track. The repository that has to be used has to be always provided by the user with a new command-line flag (--track-repository=$NAME) but the default track repository is chosen implicitly by Rally.