bin/deploy: Avoid loading merge deployment kargs unless necessary

bin/deploy: Avoid loading merge deployment kargs unless necessary

The fact that `ostree admin deploy` always itself loaded the
merge kargs masked a bug in the core.  Let's change our tests
to not pass any kernel arguments to ensure we cover this.

The new logic in the CLI is a bit subtle, but if you read
carefully is a lot clearer I believe.  Basically we have one
of a few "starting points" in the first section, which can
then be further augmented.

Closes: #1558
Approved by: jlebon