Bug 1134367 - 'Red Hat Access' open new case with jdr report throws
exception if JON is under proxy

Properly return http proxy server url (in global settings we have setting
for host/IP .. needed to prefix with http://. Also other proxy related
settings now return null if set to null or empty (our UI sets to "" as unset
value)

(cherry picked from commit 931f8549fc36abfb0fef93e474bb7e4368a7d146)
Signed-off-by: Libor Zoubek <lzoubek@redhat.com>