Merge pull request #252 from mckern/createrepo-loves-checksums

`createrepo` loves checksums! And long options!
(maint) Consolidate to methods into one

create_rpm_repo and update_rpm_repo are almost identical methods except for the
passing of --update, which will still work when creating a repo. This commit
collapses the two methods into one by aliasing create_rpm_repo to
update_rpm_repo, so that only one method needs maintenance.