Build OSSEC agent deb in app-build by default

I had previously asked James to disable building the OSSEC debs in
app-build because it was slowing down my debug cycle. However, I think
in general this is the right thing to do.
Get build_deb_packages.sh working

* Update the path to the fpf-signing-key file, which was moved
  in a refactor.
* Always create the build artifacts directory to avoid an error on first
  run
Merge pull request #740 from freedomofpress/packaging-fixes

Packaging fixes