manage_import_signatures - don't run createrepo if not needed (#647)

rhel6.0 switched to using yum natively for installs and updates, as did
the contemporary fedora. Such disks / distros already have perfectly
usable yum repodata, running createrepo is at best unnecessary, but
as noted in issue #xxx, can also destroy important data provided by
the upstream vendor.

rhel 4.x and earlier did not use yum, have no repodata on the disks,
so createrepo is needed and will be run.

This code uses a simple heuristic - if the "repodata" directory exists,
it is presumed valid and left untouched.

Tested on rhel6.4, rhel6.5, and rhel5.10, and fedora20 installation
disks, all worked as expected, and all resulted in a useable installation
tree.