Add IPv4 flag for Ironic's TFTP setup

TFTP binds to the IPv6 interface by default on Fedora, and as a result
some machines may fail PXE booting. Adding the IPv4 flag makes the TFTP
server bind to IPv4 consistently.

Change-Id: I0fdacb0d91eece8cdc52d7bc7cc76a28efaf3751
Closes-Bug: #1306554
Merge "Add IPv4 flag for Ironic's TFTP setup"