Support setting all reprepro directories + RELEASE_REPOSITORY

Do not reset REPOSITORY before running generate-reprepro-codename.
If REPOSITORY is overwritten with a release dir, it generates a new
rerepro db, and changing REPOSITORY does not take effect. If
REPOSITORY was changed to something else generate-reprepro-codename
runs into errors, because it do not know about the new overwritten
REPOSTIRORY var.

Thanks: Marcel Beck <nexeck@outlook.com> for the initial patches