Integration of snf-vncauthproxy v1.6: add TCP control socket

[salsa-dev]

Hi, devs!

Code in util/vapclient.py of the master branch says we must use the snf-vncauthproxy of the version prior to 1.5 when Unix domain control sockets were replaced with TCP control sockets: CTRL_SOCKET = "/var/run/vncauthproxy/vncproxy.sock"

Right now I'm deploying GanetiMGR (master branch) with https://github.com/grnet/snf-vncauthproxy (master, v1.6). And I'm working towards rewriting some code to support that.

Is there any specific reason for using Unix domain control socket?