Add option to disable IPv6 for load balancers

[fhofherr]

Enabling IPv6 by default in #137 broke clusters using external-dns. This
commit adds an option to disable IPv6 by setting the

load-balancer.hetzner.cloud/ipv6-disabled

annotation to true.

Closes #191

[LKaemmerling]

I would invert the annotation name, because otherwise you have Double „no“. (EnableIPv6 that defaults to true is better than having DisableIPv6 that defaults to false and when it is false it disables IPv6)

[fhofherr]

I would invert the annotation name, because otherwise you have Double „no“. (EnableIPv6 that defaults to true is better than having DisableIPv6 that defaults to false and when it is false it disables IPv6)

Only the commit message was wrong. The annotation has to be set to true of course to disable ipv6.