Expose full hostname to Changes.
Expose full hostname to Changes.

Summary:
Required by Changes to refer to a slave node (e.g., to a mesos master).
This is useful to get Changes to offline/online a slave.

Test Plan: Unit tests

Reviewers: nate, paulruan

Reviewed By: paulruan

Subscribers: changesbot, kylec

Differential Revision: https://tails.corp.dropbox.com/D179895

dropbox/changes-client,dropbox/changes-client