Add optional loadBalancerClass in service spec

[vabrams]

To use another load balancer class, we need the ability to specify LoadBalancerClass for services

[RangerRick]

@vabrams Hey, I'm taking over maintainership and going through old PRs. Just a couple of comments on this one:

1. The directory layout has gone through some rearranging since this PR was made, mind rebasing so it's pointing at the new location of service.yaml?
2. Do you mind adding the property to the values.yaml and README.md as well so it's documented alongside the other load balancer values?

Thanks!