Add `--rootless` flag to garden server
Add `--rootless` flag to garden server

The rootless flag should be used when starting the garden server as a
non-root user. Right now, it will start garden-server with nil
volumeDriver and no starters.

[finishes #130627853]

Signed-off-by: Tiago Scolari <f438736fd7dd1cb07f8d2a37f4499d0a0272a752@pivotal.io>

cloudfoundry-incubator/guardian,cloudfoundry-incubator/guardian,cloudfoundry-incubator/guardian