Put vagrant sudo configuration in a separate file

Put vagrant sudo configuration in `/etc/sudoers.d/vagrant` rather than `/etc/sudoers`, so that if, say, Chef writes out a new `/etc/sudoers`, sudo from vagrant continues to work.

Other platform templates already do this.  It's handled a bit inconsistently at the moment.  This is just the platform I was interested in.

Note, I don't know how all the pieces fit together.  There is similar code in `packer/scripts/centos/ks.cfg`, but I don't know if anything uses that.