Makefile: run `apt-get update` before installing packages

Sometimes travis builds fail, because apt local index are outdated.
Merge pull request #91 from freyes/travis-apt-get-update

Makefile: run `apt-get update` before installing packages
mkcloud: add update steps to 'plain' target

This patch makes `plain` target consistent with `all_noreboot` and `all`
mkcloud: sync doc of how commands all and all_noreboot

This patch documents that `all` and `all_noreboot` run `addupdaterepo`
and `runupdate` steps.