Clean up leaked images and FaaS deployments

[rjew]

Fixes #564, Fixes #219

• Clean up images from docker host when updating/deleting functions, images, and base-images
• FaaS function deployments are now deleted when a function is updated

There are still some images being leaked when creating a function - the function manager pulls down "dispatch images" when creating a function. Deleting these could cause a race condition if there is also a function being created at the same time which depends on that image.

TODO Delete images from the docker registry when a function/image is deleted

[rjew]

Sure I'll work on adding one.

[rjew]

If someone tries to exec a function while it is being deleted, it shouldn't work since a function will only run if it is in status READY so the order shouldn't matter.