debian: cowbuilder doesn't seem to honour EXTRAPACKAGES with --create

cowbuilder --create seems to ignore the EXTRAPACKAGES configuration
option.   cowbuilder --update doesn't, so we run --update after --create
to make sure apt-utils is installed in the chroot.

This seems like a bug, either in cowbuilder or in the way we are using it.

Signed-off-by: Euan Harris <euan.harris@citrix.com>