Use libcloud methods to set labels

[tewfik-ghariani]

Labels applied to GCE instances were added before there was any method available in libcloud

https://github.com/nix-community/nixops-gce/blob/d56aa7b68557e6fd2661c90be89d70419d9393d1/nixops/backends/gce.py#L478-L479

Now that https://github.com/apache/libcloud/pull/1115 is merged, we need to modify our code to take advantage of ex_set_volume_labels, ex_set_image_labels, ex_set_node_labels methods

Ref : https://github.com/nix-community/nixops-gce/commit/d56aa7b68557e6fd2661c90be89d70419d9393d1