Replace x11vnc with tightvnc.

I'm running my docker-selenium nodes using tightvnc to record video of browser sessions over the network. I've found it to be more responsive than x11vnc.  If there is interest in keeping x11vnc the entry point could be modularized so that either x11vnc or tightvnc could be started, e.g. `VNCSERVER=[x11vnc | tightvnc]`