Fix vncproxy package/service name on Debian

This commit fix vncproxy on debian os, we have now a duplicate
between nova::vncproxy and nova::spicehtml5proxy packages on debian
but install both have no sense…

Closes-bug: #1267645

Change-Id: Ib0a769ba91221d1ebc14df01f24579c8fe58db26
(cherry picked from commit c5eb65b0ad498eff4928d95e2dba31807844b5c0)