Accomplish fix #1027: move backend/debian into debian/.

Move debian directory top-level, plus ignore all irrelevant files from source,
and update appliance scripts accordingly.
XXX. globaleaks.deb shall be in common for all these appliances scripts, in a
separate file under appliance/.
Update .gitignore with respect to latest commit.
Introduce packer user variable `deb`.

The introduction of this variable is due to the fact that packer does not report
in the official documentation any way to guess globaleaks version, and find a
globaleaks debian package automagically. Therefore, from now on the user will
have to specify this variable pointing to the globaleaks .deb file generated by
dpkg. E.g.:

$ packer build -var deb=../../../globaleaks_2.60.30_all.deb template.json