SetUserSSHPublicKeysPlugin needs to use
the metadata service provided admin username (if provided),
to be consistent with the other plugins.
Fixes https://github.com/cloudbase/cloudbase-init/issues/22
Co-Authored-By: Rui Lopes <rgl@ruilopes.com>
Change-Id: Ie0df53a8aaaa15e9c82c620159bc846bca649b1b