capture output from sudo beaker-clear-netboot in case it fails

Also added logging for uncaught exceptions in beaker-proxy request
handling, since currently these go to stderr which is /dev/null when
daemonized.

Bug: 1003709
Change-Id: Ide61e64ef8fd359ea032cb60995fd466e9670a9b
list lab controllers separated by newlines

Based on a patch contributed by Marian Ganisin.

Bug: 1009903
Change-Id: I2aa4043d24a23b87a60edba38be6095244da322d