Consider moving Compute's ServiceAccount to core

[mziccard]

Compute instances contain a list of service accounts, each of each has an email and scope fields. We should consider moving this information to gcloud-java-core as soon as it might be needed elsewhere.

[garrettjonesgoogle]

This has been added to our feature backlog: https://github.com/GoogleCloudPlatform/google-cloud-java/wiki/Feature-backlog . This issue will be closed but is linked in the backlog and can continue to be used for comment and discussion.