Use GCE OS image families to get latest image version

[mbrukman]

provider/src/main/resources/com/cloudera/director/google/google.conf includes the following snippet:

google {
  compute {
    imageAliases {
      centos6 = "https://www.googleapis.com/compute/v1/projects/centos-cloud/global/images/centos-6-v20160526",
      rhel6 = "https://www.googleapis.com/compute/v1/projects/rhel-cloud/global/images/rhel-6-v20160511"
    }
[...]

which is hard-coding a specific image versions of CentOS 6 and RHEL 6, respectively, but both are from 2016. They're both quite old, so they should be updated to newer versions. However, it does not need to be manual work every time: using image families, one can get the benefit of these image aliases always pointing to the latest version, without having to have an explicit alias in such a configuration:

Image families simplify the process of managing images in your project by grouping related images together and making it easy to roll forward and roll back between specific image versions. An image family always points to the latest version of an image that is not deprecated. Most public images are grouped into an image families. For example, the debian-8 image family in the debian-cloud project always points to the most recent Debian 8 image.

[jadair-cloudera]

Looks like a great improvement.