Provide stack manager flag not to pull images

[sm453]

Current behaviour: When starting a stack, the stack manager tries three times to pull each image. If these requests all time out, it may take up to an hour to spin up a stack. This is makes it unusable in practice.

Suggestion: Allow the user to pass an optional flag that suppresses image pulling altogether. Only locally available images should be used, and if no suitable image can be found, it should fatally terminate.