Parameterize the stack image location

[robdimsdale]

Currently switchblade makes an assumption that the stack lives in the cloudfoundry org. That isn't always true. We should parameterize this so we can use stacks from other orgs. We can still default to the cloudfoundry org if we wish.

[robdimsdale]

Addressing this issue would also allow switchblade to be used for testing stacks too - you could point switchblade to e.g. a newly-created stack on the local docker daemon.