Add the option to not automatically --rm docker images and auto pull on every nebula app change

[naorlivne]

not automatically removing images & repulling them might have some uses in cases where you want to reuse local based images, when this is added there should also be a way to force\order GC older images that are unused (one of the original reason why currently Nebula automatically deletes all images so aggressively) otherwise I can imagine IoT devices will get filled with old images quickly.

[naorlivne]

the new /prune option added will provide option to clean unused images so all hat remains of this is to have an option not to set force_pull = False in the app config

[naorlivne]

rethinking about it made me realize that having a no pull option might lead to risky usages (what if someone cleans some of the devices of images between run but not all of them? what if someone pushes a new version over the same tag?) and it is a lot safer to always repull images, seeing how if the image layer is already in the drive it doesn't really forces downloading all over it again the time it takes for the extra pull is already minimal and with the new /prune endpoint there shouldn't be much worries about disk space anyway.