backendVdcs: fix errors without backend

Fixes errors in log:

[2018-08-29T12:38:20.917+02:00]   972 NOTIC backendVdcs BackendVdcs .cpp:56
[2018-08-29T12:38:23.759+02:00]   972 NOTIC backendVdcs Http request x.url:https://private-dev.devapi.digitalstrom.com/v1/cloudvdc/vdcs code:401 result:{"message":"Non-ambiguous authorization tokens are missing."} elapsed:2831ms .cpp:387
[2018-08-29T12:38:23.765+02:00]   972 ERROR backendVdcs Failed to update vdcs e:Http request failed. x.url:https://private-dev.devapi.digitalstrom.com/v1/cloudvdc/vdcs code:401 result:{"message":"Non-ambiguous authorization tokens are missing."} condition:code / 100 == 2 backend-vdcs.cpp:388 .cpp:333

https://trello.com/c/S8Rwlp9X/183-bug-wrong-ds-hub-private-url-on-dss11-1gb

https://digitalstrom-zrh.atlassian.net/browse/DSD-58