Unmount old shared data dir and all children
Unmount old shared data dir and all children

This works for the two scenarios that are possible:
- rep + garden without bpm where the old shared directory was never
mounted but its child instance identity dir tmpfs was
  - If we look for `/var/vcap/data/rep/shared_data` in this case we will find the
  tmpfs mount but try to unmount its parent which will fail so we simply
  find all mounts under that old directory and unmount them individually
  so we dont try to unmount something that isnt mounted
- rep + garden with bpm where the old shared directory and the child
instance identity tmpfs were both mounts

[#161177249](https://www.pivotaltracker.com/story/show/161177249)

Signed-off-by: Sunjay Bhatia <df2d57be7987a471915f625a5499155bc16d4200@pivotal.io>

cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release