We need the ability to explicitly disable ipv6 or ipv4 on a test host. That way we can make sure that the communication will be done over v4 or v6 in a multihost set up. Also we want to know what protocol we'll be using when querying DNS, pulling packages from repositories, sending the results, etc.
Note that it's not mandatory to provision the host over ipv6 due to many limitations of PXE, and other beaker components - there will be another request for that.
I already have a bz open for adding ipv6 support to beah.
As far as enabling/disabling ipv6/ipv4 on a host its outside of beaker. You pass options to anaconda noipv4 or noipv6 for installation.
if you install with noipv4 anaconda should not configure the network to use ipv4 post install either.