replace hard coded url for vnc view

The URL for VNC view is hard coded to /project/instances/[instance_id]/vnc,
which will fail, if the dashboard is not placed in the root of the
web server.

Fixes bug 1228033

Change-Id: Ib0bb76f03b3adf6bd0d85230a292dec62298ecc4
Merge "replace hard coded url for vnc view"