Change root path for stage_config_path and deploy_config_path (refs #1060)

Like described in #1060 

Defaults for stage_config_path and deploy_config_path have been relative paths.
When I'm in a subfolder, for instance lib, and want to call cap <stage> deploy
it does not work because theses paths are relative to my working directory. Rake supports this kind of behavior so I think capistrano should support
this feature too.

This PR changes the behavior of stage_config_path and deploy_config_path and set boot relative to the capfile as default.