(maint) Use GNU-style long flags for createrepo

While updating `createrepo` commands to use SHA1 checksums as an option,
I also replaced usage of short flags with GNU-style long flags. Is it
more characters? Absolutely. Is intent a little bit clearer now? I hope so.
(RE-691) Add '--checksum=sha' to createrepo commands

This change is needed to support the pending upgrade of `createrepo`
from 0.4.11 to 0.9.9. If you use the `--checksum` flag with 0.4.11,
it complains to STDERR about a deprecated option but it doesn't stop
repo scanning. If we stage this change now, then legacy yum instances
(such as EL4) will continue to work when we upgrade `createrepo`.
(maint) Use an intermediate variable for remote command in :update_yum_repo

This brings usage more in-line with other instances of `remote_ssh_cmd`, and
will make it easier to break the command apart in the future.
(maint) Split up the remote:update_yum_repo `createrepo` command

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

`createrepo` loves checksums! And long options!