Fix Centos Initramfs Grow Root setup

Currently the centos packer script downloads a git repo in order to manage initramfs and cloud-init-growroot.

There is a dracut module that handles this without having to install git, clone a repo, and create a seperate initramfs.

It just installs ```dracut-modules-growroot``` and then tels dracut to recreate the initramfs.

Ihave tested this packer script and it grows the root partition without any issues.