URLs for CentOS ISOs are not working. Replaced with recommended mirror.

Direct links to ISOs on vault.centos.org redirect to http://vault.centos.org/notonvault.html with instructions to use mirror.symnds.com for ISO downloads. 

Updated all references to vault.centos.org ISOs, but left vault.centos.org for supporting files (these resources are still available on vault.centos.org). 

I *did* individually check that all the ISO links work, but I did *not* build and test images for each of the ISOs using veewee. If there is a recommended way to do this I can give it a try, but just switching out the ISO URLs with a mirror hopefully won't require that level of testing.