We currently use the hostname of the builder host as the
identifier for the owner of built images. This value is
recorded in ZooKeeper and is used to make sure that only
the owner of an image build ever deletes the entry from
ZooKeeper.
Using hostname can be problematic if it ever changes. It will
cause orphaned builds that will not get deleted. This change
allows us to use a UUID as the identifier, deprecating use of
hostname altogether (although we continue storing that info).
The UUID will be stored in a file in the images directory so
that it may persist across nodepool-builder restarts.
In order to help with transitioning existing builders to using
UUID instead of hostname, the code will always compare the UUID
value AND the hostname so that existing ZK entries will be
matched until they age away.
Change-Id: Ifafbab9fb0f41564cc1af595586fa7353ce1d0d0
fcc68b8fd6