added local yum repo hosting on puppet server when started as centos 

for Centos hosts can put RPM files into repository/yum/ and then run `vagrant provision puppet` to which will kick off a `createrepo` to add the RPM into the local repo.   centos clients get a sandbox.repo file to auto-point to the yum repo.


also flipped the vagrantfile into a symlink so can easily flip between OS choices.