Previously we always used a random short_id, and that is indeed what we
want for Autoscaling groups, but ResourceGroups generally use sequential
IDs, so pass a function that generates them instead of hard-coding it.
By happy accident, this makes the tests a bit nicer too.
Change-Id: I58ce67e4b58378f35d5690d653a113b03ee6e08e
Partially-Implements: blueprint scaling-group-common