sign_and_index_rpm_repo: speed up createrepo indexing

Run "createrepo" with the --update and --checkts options. This allows
createrepo to skip a lot of the metadata generation. With thousands of released
RPMs on ceph.com, every bit helps.

Signed-off-by: Ken Dreyer <ken.dreyer@inktank.com>
Merge pull request #9 from kdreyer-inktank/createrepo

sign_and_index_rpm_repo: speed up createrepo indexing