installer: Allow host-bin restoration only.

Add a  `-b` option to the installer that only restores host-bin scripts, based on the target files installed in all existing chroots. This happens to be a prerequisite for #706 .

Incidentally, this fixes #1106 as crouton -f tarball now only restores host-bin (and not chroot-bin, which would mangle croutonversion output, see https://github.com/dnschneid/crouton/issues/1106#issuecomment-59576779).

We also provide a minimal test for `-b`: binaries are restored.

I've read somewhere that you need to have a failing test before fixing issues. This confirms #1106:
`2014-10-20_03-49-08_drinkcat_chroagh_splittest+edit-chroot-fix.testonly_12` (only e62f4b1)

Then a full test:
`2014-10-20_06-01-54_drinkcat_chroagh_splittest+edit-chroot-fix`
another one that tests all targets (`x0`):
`2014-10-20_06-02-35_drinkcat_chroagh_splittest+edit-chroot-fix_x0` (mirrors are flaky, but overall it's ok)
and a last one with the latest cosmetic changes:
`2014-10-20_17-13-16_drinkcat_chroagh_splittest+edit-chroot-fix_12`
I was thinking that we don't want to advertise the target before we are sure it is installed...
On the other hand, in case of failure, the user will be reminded to run the prepare script again, so your approach should be good.