Add flag to skip cleanup on failed deploy

[lovro-bikic]

Mina by default cleans up everything in the build path. This is okay for successful deploys, but it can be a problem for failed deploys because it doesn't allow inspection (e.g. if you want to ssh to the server to see what failed exactly).

Add a variable (maybe :skip_cleanup_on_fail) which when set to true will skip the cleanup. It should be false by default because we don't want to introduce a breaking change.