Don't install novnc from package by default

By default the installation is failed because the novnc package has got
the a dependency of the nova-common package. The last package provides
"/etc/nova/*" files with appropriate privileges. It potentially brings
to the problem of an access to those files.

Change-Id: I1689bef817365b10bf972dc4a8033892ad5939d1
Related-Bug: 1248923