openqa-bootstrap: Allow curl to follow redirects (boo#1245379)

We're mainly interested in the http response code but want to follow
eventual redirects. To follow these redirects we can use curls
`--location`/`-L`-option but this will cause multiple responses (until
no redirect is left). Since `--head`/`-I` will always print all headers
for all responses, we now need to redirect *everything* to /dev/null
and print the actual, last response code with `--write-out`/`-w` only.

Author: nicksinger

script/openqa-bootstrap

@@ -163,7 +163,7 @@ grep -q "$(hostname)" /etc/hosts || echo "127.0.0.1 $(hostname)" >> /etc/hosts
 start-daemons
 
 # wait for webui to become available
-while ! curl -sI http://localhost/ | grep 200; do
+while ! curl -o /dev/null -w "%{http_code}" -sIL http://localhost/ | grep 200; do
     sleep 3
 done