Make tests pass on Jenkins!

* Quit using NFS shared folders on Jenkins, since they don't work at the moment.
* Enable symlinks on the dxr shared folder so trilite will build.
* Make many tweaks to the Jenkins config:
  * Build our vagrant box manually rather than with the Jenkins plugin. Run the tests over ssh from a normal "execute shell" build step as well, since it doesn't find our manually-created box otherwise.
  * Get the path to vagrant executable right.