Add support for the "prefix=" option

Please note that the "esxi5" template has been modified to include the missing "scsi-rst.v00" module that was added in the ESXi 5.0.0 U1/U2 releases.

Unfortunately this will limit template support to ESXi 5.0.0 U1/U2 only: importing ESXi 5.0.0 will succeed but initial deployment will not boot due to a missing "scsi-rst.v00" file not present on that release's ISO image but referred to in the cobbler-boot.cfg TFTP file.

Unless import detection can be added for the various 5.0.0.x releases, I would recommend support for the initial ESXi 5.0.0 release be dropped as ESXi users should really be on the U1/U2 release for security/bug/performance reasons.

Other suggestions welcome.