Fix styles for error page template
Fix styles for error page template

500.html template has incorrect styles for OpenStack logo. This patch
fixes margins and background image size to fit into the container.

Change-Id: Icc2af2731eb8a842c12c073474f3865709b6de2b
Closes-Bug: #1696437

noironetworks/horizon,openstack/horizon,noironetworks/horizon,ChameleonCloud/horizon,BiznetGIO/horizon,yeming233/horizon,yeming233/horizon,ChameleonCloud/horizon,BiznetGIO/horizon,yeming233/horizon,openstack/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,openstack/horizon,noironetworks/horizon,BiznetGIO/horizon,NeCTAR-RC/horizon,yeming233/horizon,BiznetGIO/horizon,NeCTAR-RC/horizon,noironetworks/horizon,NeCTAR-RC/horizon,openstack/horizon,ChameleonCloud/horizon