* tests: cover auth.json NFS related fixes in nfs-support test
* client,tests: do not try to change the gid when reading/removing auth.json
the group is not relevant for this even under NFS, otoh
trying to switch it fails under running with sg
this makes sg lxd -c "snap..." work again
* skip nfs-support in ubuntu trusty
group 'systemd-journal' does not exist
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Co-authored-by: Sergio Cazzolato <sergio.cazzolato@canonical.com>