Image plugin takes priority over garden-shed
Image plugin takes priority over garden-shed

Basically don't do any of the shed-related setup if an image plugin
has been provided.

[#152470748]

Signed-off-by: Ed King <cb0ef010d3b7764b76e01d19f7526ae755120a2b@pivotal.io>

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