pg_createsubscriber: reword dry-run log messages

pg_createsubscriber: reword dry-run log messages

The original messages were confusing in dry-run mode in that they state
that something is being done, when in reality it isn't.  Use alternative
wording in that case, to make the distinction clear.

Author: Peter Smith <[email protected]>
Reviewed-by: Chao Li <[email protected]>
Reviewed-by: Euler Taveira <[email protected]>
Backpatch-through: 18
Discussion: https://postgr.es/m/CAHut+PsvQJQnQO0KT0S2oegenkvJ8FUuY-QS5syyqmT24R2xFQ@mail.gmail.com