make bind option actually work - ie. do not set  ipaddress per default

Using the new bind option, makes haproxy frontend and listen complain about ipaddress AND bind being set.. because ipaddress is per default.

I set it to undef per default, and then set it to the default value ($::ipaddress), ONLY if $bind was not set and no $ipaddress argument was given - so old behavior is kept.
Causes `Cannot reassign variable ipaddress` compilation error.