Promote btrfs
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
upstart: use exec here so upstart can monitor the process and not just a shell

Docker-DCO-1.1-Signed-off-by: Michael Gorsuch <michael.gorsuch@gmail.com> (github: gorsuch)
Merge pull request #4838 from crosbymichael/btrfs-ftw

Promote btrfs
Merge pull request #4879 from gorsuch/exec-upstart

Use "exec" in the upstart script to let upstart manage the docker process instead of a shell