Update documentation to clarify Image Upload setting
Update documentation to clarify Image Upload setting

The documentation at http://docs.openstack.org/developer/horizon/topics
/deployment.html#file-uploads states that the end user should change the
setting "HORIZON_IMAGES_ALLOW_UPLOAD = False" to disable image uploads.

This suggests that image uploads will be completely disabled, however it
will still be possible to upload images via URL.

This change adds this clarification to the documentation.

Closes-Bug: 1215921
Change-Id: Ie1cc51aaf459f9eb2881121b405fb16e762e265b

NeCTAR-RC/horizon,redhat-cip/horizon,noironetworks/horizon,dan1/horizon-proto,mdavid/horizon,mdavid/horizon,tellesnobrega/horizon,mrunge/openstack_horizon,gerrive/horizon,Tesora/tesora-horizon,maestro-hybrid-cloud/horizon,luhanhan/horizon,froyobin/horizon,yjxtogo/horizon,wolverineav/horizon,doug-fish/horizon,openstack/horizon,mrunge/horizon_lib,liyitest/rr,Hodorable/0602,icloudrnd/automation_tools,django-leonardo/horizon,icloudrnd/automation_tools,saydulk/horizon,mrunge/horizon,ChameleonCloud/horizon,karthik-suresh/horizon,Dark-Hacker/horizon,NCI-Cloud/horizon,gerrive/horizon,openstack/horizon,tellesnobrega/horizon,django-leonardo/horizon,RudoCris/horizon,BiznetGIO/horizon,Solinea/horizon,tsufiev/horizon,Mirantis/mos-horizon,aaronorosen/horizon-congress,idjaw/horizon,ChameleonCloud/horizon,henaras/horizon,bac/horizon,izadorozhna/dashboard_integration_tests,ging/horizon,django-leonardo/horizon,newrocknj/horizon,RudoCris/horizon,redhat-cip/horizon,maestro-hybrid-cloud/horizon,Mirantis/mos-horizon,gerrive/horizon,saydulk/horizon,mrunge/horizon,VaneCloud/horizon,coreycb/horizon,RudoCris/horizon,RudoCris/horizon,flochaz/horizon,karthik-suresh/horizon,promptworks/horizon,Metaswitch/horizon,kfox1111/horizon,philoniare/horizon,redhat-openstack/horizon,openstack/horizon,bac/horizon,wolverineav/horizon,eayunstack/horizon,agileblaze/OpenStackTwoFactorAuthentication,VaneCloud/horizon,damien-dg/horizon,pranavtendolkr/horizon,dan1/horizon-proto,newrocknj/horizon,blueboxgroup/horizon,NCI-Cloud/horizon,Solinea/horizon,nvoron23/avos,Daniex/horizon,flochaz/horizon,aaronorosen/horizon-congress,zouyapeng/horizon,redhat-openstack/horizon,Dark-Hacker/horizon,yjxtogo/horizon,luhanhan/horizon,Solinea/horizon,idjaw/horizon,liyitest/rr,dan1/horizon-x509,django-leonardo/horizon,JioCloud/horizon,pranavtendolkr/horizon,promptworks/horizon,FNST-OpenStack/horizon,wangxiangyu/horizon,mrunge/openstack_horizon,endorphinl/horizon-fork,NeCTAR-RC/horizon,sandvine/horizon,icloudrnd/automation_tools,mrunge/horizon_lib,xinwu/horizon,tellesnobrega/horizon,blueboxgroup/horizon,nvoron23/avos,JioCloud/horizon,Metaswitch/horizon,anthonydillon/horizon,icloudrnd/automation_tools,FNST-OpenStack/horizon,bac/horizon,froyobin/horizon,tellesnobrega/horizon,anthonydillon/horizon,agileblaze/OpenStackTwoFactorAuthentication,bigswitch/horizon,Dark-Hacker/horizon,yeming233/horizon,yjxtogo/horizon,dan1/horizon-x509,xme1226/horizon,yeming233/horizon,NeCTAR-RC/horizon,CiscoSystems/horizon,bac/horizon,orbitfp7/horizon,pranavtendolkr/horizon,agileblaze/OpenStackTwoFactorAuthentication,aaronorosen/horizon-congress,damien-dg/horizon,BiznetGIO/horizon,ChameleonCloud/horizon,promptworks/horizon,takeshineshiro/horizon,philoniare/horizon,newrocknj/horizon,tqtran7/horizon,orbitfp7/horizon,bigswitch/horizon,sandvine/horizon,karthik-suresh/horizon,anthonydillon/horizon,promptworks/horizon,coreycb/horizon,zouyapeng/horizon,sandvine/horizon,BiznetGIO/horizon,redhat-openstack/horizon,nvoron23/avos,kfox1111/horizon,ging/horizon,xme1226/horizon,Tesora/tesora-horizon,Metaswitch/horizon,endorphinl/horizon,takeshineshiro/horizon,Tesora/tesora-horizon,davidcusatis/horizon,davidcusatis/horizon,idjaw/horizon,saydulk/horizon,tsufiev/horizon,noironetworks/horizon,henaras/horizon,Hodorable/0602,coreycb/horizon,flochaz/horizon,davidcusatis/horizon,endorphinl/horizon-fork,davidcusatis/horizon,redhat-cip/horizon,endorphinl/horizon,mdavid/horizon,endorphinl/horizon,xinwu/horizon,tsufiev/horizon,ging/horizon,maestro-hybrid-cloud/horizon,wangxiangyu/horizon,Daniex/horizon,dan1/horizon-proto,tqtran7/horizon,gerrive/horizon,anthonydillon/horizon,tsufiev/horizon,mrunge/openstack_horizon,endorphinl/horizon-fork,philoniare/horizon,xinwu/horizon,endorphinl/horizon-fork,mandeepdhami/horizon,pranavtendolkr/horizon,mdavid/horizon,vladryk/horizon,FNST-OpenStack/horizon,orbitfp7/horizon,Hodorable/0602,Mirantis/mos-horizon,wolverineav/horizon,xinwu/horizon,henaras/horizon,tqtran7/horizon,Metaswitch/horizon,endorphinl/horizon,Mirantis/mos-horizon,watonyweng/horizon,saydulk/horizon,redhat-openstack/horizon,idjaw/horizon,mandeepdhami/horizon,zouyapeng/horizon,luhanhan/horizon,blueboxgroup/horizon,tqtran7/horizon,VaneCloud/horizon,VaneCloud/horizon,izadorozhna/dashboard_integration_tests,JioCloud/horizon,vladryk/horizon,maestro-hybrid-cloud/horizon,yeming233/horizon,ging/horizon,NCI-Cloud/horizon,doug-fish/horizon,vladryk/horizon,dan1/horizon-proto,CiscoSystems/avos,noironetworks/horizon,dan1/horizon-x509,kfox1111/horizon,Dark-Hacker/horizon,newrocknj/horizon,yeming233/horizon,froyobin/horizon,damien-dg/horizon,yjxtogo/horizon,watonyweng/horizon,CiscoSystems/avos,doug-fish/horizon,Solinea/horizon,openstack/horizon,NCI-Cloud/horizon,Tesora/tesora-horizon,mandeepdhami/horizon,CiscoSystems/horizon,Daniex/horizon,damien-dg/horizon,takeshineshiro/horizon,mrunge/horizon_lib,CiscoSystems/horizon,blueboxgroup/horizon,sandvine/horizon,dan1/horizon-x509,bigswitch/horizon,nvoron23/avos,j4/horizon,vladryk/horizon,bigswitch/horizon,mrunge/horizon,eayunstack/horizon,wangxiangyu/horizon,coreycb/horizon,wangxiangyu/horizon,orbitfp7/horizon,j4/horizon,zouyapeng/horizon,luhanhan/horizon,NeCTAR-RC/horizon,eayunstack/horizon,mandeepdhami/horizon,j4/horizon,watonyweng/horizon,doug-fish/horizon,xme1226/horizon,liyitest/rr,Hodorable/0602,CiscoSystems/avos,CiscoSystems/avos,karthik-suresh/horizon,henaras/horizon,agileblaze/OpenStackTwoFactorAuthentication,Daniex/horizon,wolverineav/horizon,CiscoSystems/horizon,kfox1111/horizon,noironetworks/horizon,watonyweng/horizon,philoniare/horizon,FNST-OpenStack/horizon,ChameleonCloud/horizon,liyitest/rr,takeshineshiro/horizon,j4/horizon,redhat-cip/horizon,flochaz/horizon,BiznetGIO/horizon