Prevent tftp sysv init script from firing.

This is an equivalent fix to I01f34c6869b60432cff115e46246cd526ec12a62
for ironic; it disables any SysV startup script from launching
tftpd before xinetd can get a look-in.

Change-Id: I263aa756f80f7cbcbf5ca5ef5226136e9eab7ba1
Merge "Prevent tftp sysv init script from firing."