[GCE] Propagate labels to separate gce disks during creation

[tewfik-ghariani]

Hello all,

It has been brought to my attention that the disks labels are not being propagated at all when creating new separate gce-disks. This behavior has been fixed for ebs volumes as tags are inherited correctly but it's not the case for gce

EC2 : https://github.com/NixOS/nixops/blob/master/nixops/resources/ebs_volume.py#L156
GCE : https://github.com/NixOS/nixops/blob/master/nixops/resources/gce_disk.py#L113

Need to enable the automatic inheritance of the labels from the associated machine

[tewfik-ghariani]

@adisbladis can this be moved to nixops-gce?