autoinstall: disable multiarch by default

multiarch is set to i386 on amd64 systems by default since apt-setup
1:0.79, i.e. Ubuntu saucy. We don't need it, so disable it.

Change-Id: Id2c4d52617a05776c40dcc900dda4125e06e6b6d
autoinstall: set up apt pinning from early in d-i

We currently have the need that we need to install a lower version of
puppet (2.7 instead of 3.4) from our own apt repository in trusty, but
we have no apt pinning set up while in the installer, and hence when
both are set at 500, the highest version wins.

Unfortunately, apt-setup does not support setting up apt pinning and
both early_command & late_command are unsuitable for doing so (the
former runs before we have /target, the latter runs after apt-get update
has been run).

Work around this in a very hacky way, by having the early_command shell
script install a shell script hook under base-installer's
prehooks runpartsdir, which runs before the apt_update step. It also
runs before the install_base_system step and there's no hook step
between those two, but this should be okay as long as we mkdir
/etc/apt/preferences.d ourselves.

Change-Id: I6dcb9d340aa54f52cdb96ef3cbcc51628e4001d9