autoinstall: another trusty early_command fix

killall.sh for killing DHCP was (rightfully) renamed to kill-all-dhcp to
indicate its purpose in netcfg 1.86. wheezy has 1.108 & trusty has
1.114 and are the first versions to have this change.

Add support for in a backwards-compatible way and while at it, remove a
useless "true" invocation.

Change-Id: I282f8e2d7187e98b8e544335baba9501463b7570
autoinstall: move disable_dhcp to early_command

There is no reason for the disable_dhcp/disable_autoconfig boilerplate
to be present in every single subnet definition where it can be moved in
the place where we actually set the static IP address and call netcfg
itself. This has the downside that it to a shell script, but it has the
benefit of DRY, plus that we can document all netcfg exceptions.

While at it, remove the netcfg/choose_interface setting from a couple of
subnets, as it's defined in common.cfg anyway.

Change-Id: I72681530ab76211e96a45ee46acb6e2b177e36e4