Add support for making update_hostname optional.

In trying to make Alpine Linux our new base image on EC2 we needed to prevent tiny-ec2-bootstrap from modifying the hostname during bootstrapping. We have some legacy cases where we handle this in our images via a DHCP exit hook and it is integrated with some additional internal tooling. I thought there may be other uses cases like this out there and hope that the change can just get merged upstream and avoid having to maintain the customization locally. This would make the update_hostname function optional driven by a new environment variable that defaults to a value of yes which maintains the existing behavior. The behavior would only be changed by an end user explicitly setting it to value of no in conf.d.