vpnkit-expose-port: add a viper config file
vpnkit-expose-port: add a viper config file

This allows the control-vsock to be set somewhere and the value to
be read when dockerd invokes vpnkit-expose-port

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit