fixed outdated reference to novnc

The cloud admin guide contained these
out-of-date commands:

apt-get install novnc
service novnc restart

I changed them to the current syntax,
which is:

apt-get install nova-novncproxy
service nova-novncproxy restart

Change-Id: Ib4e2d79951f6a122535dd0fbe7134e6cca59b494
Closes-Bug: 1394345
Merge "fixed outdated reference to novnc"