Added librato backend, Fedora support, default_release option moved to variables

Hi!

Several changes are made:

1. Added librato backend support. Graphite backend is still enabled by default to keep backwards compatibility, but it's possible to disable it by setting graphite_host = ''.

2. Added Fedora support (init script + conditions in tasks)

3. default_release option of apt now takes value from variables (default is *backports)

All changes in the same pull request because it's not possible to merge it one by one (all changes the same code in tasks)

P.S. Thanks for sharing this role!