[connman] add configurable online status url

With this patch, there is no default url, so you need to add something
in main.conf:
Ipv4StatusUrl = http://someurl.com/status.html
Ipv6StatusUrl = http://someurl.com/status.html

default connman's url are at:
http://ipv4.connman.net/online/status.html
http://ipv6.connman.net/online/status.html
even if nothing is set in config file