core: ovirt-websocket-proxy fail when missing enforced key/pair

This patch makes websocket proxy fail when SSL_ONLY is set to True and
key or certificate file is missing.

Change-Id: I47033df4ea2c12799bb8535e1ef749967c2b4d06
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1116393