Add shared volumes support

Add support for shared and isolated volumes. Fix https://github.com/globocom/tsuru/issues/713

Configuration changes:

    docker:
      sharedfs:
        hostdir: /path/to/basedir/on/docker/node
        mountpoint: /mountpoint/inside/container
        app-isolation: true # if TRUE, hostdir will be isolated by appname, like: /path/to/basedir/on/docker/node/APPNAME/. Default to false

This removes the need for FUSE (https://github.com/globocom/tsuru/issues/713) and allow a shared filesystem like GlusterFS, S3QL or anything else that is mountable in docker node, to be accessed in RW by each container.