Remove the HTS_COPY_VNC_PASSWORD constant/feature

Currently just for xen-hvm we copy the vnc password on node-add.  This
will be changed for 2.1 with a more advanced gnt-cluster redist-conf
functionality which is going to be used by node-add as well.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>