Add --replace flag for load, start and submit

[klausenbusk]

This commit allow people to "replace" a unit in the registry, even through it already exits.

Dunno if it works, I haven't tested it, also I need some help with the description of the --force option.

Fix #760

[antrik]

This is an interesting approach. All conversation and pull requests regarding this so far have been about adding a separate command -- but adding a --force flag to the existing commands might indeed be preferable?

[klausenbusk]

Closed in favor of #1509 by @tixxdz

[tixxdz]

@klausenbusk thank you we used the same "--replace" switch but it needed more work.