[connman] vpn: Do not start connman-vpnd automatically

To save memory for most users, disable connman-vpnd D-Bus activation
(connmand vpn plugin will issue a net.connman.vpn call at startup)

The daemon can still be started and stopped with systemctl after the
changes.
Merge pull request #29 from hmallat/do-not-start-vpnd-on-bootup

[connman] vpn: Do not start connman-vpnd automatically