Support setting custom aptoptions for FAI

Support setting custom aptoptions for FAI

/dev/pts might not be available, for for example when running grml-live
inside a (unprivileged) docker container. Then apt/aptitude invocations
cause the following error message to show up:

| E: Can not write log (Is /dev/pts mounted?) - posix_openpt (19: No such device)

Sadly FAI's task_instsoft include the following hardcoded check:

| grep -E "^E:" $LOGDIR/software.log && task_error 472

So whenever the /dev/pts error message shows up, FAI considers this as
error and we're failing installation.

apt supports usage of `-o Dpkg::Use-Pty=0` though to not trigger the
pty related code (pkgDPkgPM::StartPtyMagic()).

By setting the environment variable aptoptions="-o Dpkg::Use-Pty=0" for
FAI execution, we can avoid this error message showing up in FAI's
software.log and being considered as error.

Support passing custom environment variables to the fai command line via
$FAI_ENV.