add additional mutable distro: fedora/centos

[Brandon B. Jozsa]

i want to start the work for adding additional mutable distros, beyond just ubuntu which is available today:

• centos
• fedora

This means that the following files need to be modified/added:

• gantry command/script needs modification.
• a new prep script needs to be created in the distro directory.
• potentially separate scripts for mutable/immutable distros in bin directory that gantry calls.