Use ubuntu keyserver for installing vagrant repo (#267)
Use ubuntu keyserver for installing vagrant repo (#267)

The previously used `pgp.mit.edu` keyserver did not respond. Changed to use the ubuntu keyserver, as suggested by the bash snippet on the official website of the unofficial vagrant debian repository (https://vagrant-deb.linestarve.com/).
Also added an error check to avoid continuing installation without this repo (as that leads to the wrong version of vagrant being installed).
netsec-ethz/scion-coord,netsec-ethz/scion-coord,netsec-ethz/scion-coord,netsec-ethz/scion-coord,netsec-ethz/scion-coord,netsec-ethz/scion-coord