Use UNSET_VALUE for @args in shell config since [] is truthy.
Fix wrong reference to Errors::NFSNoHostonlyNetwork introduced in 022c1da [GH-1437].
Merge pull request #1448 from uggedal/fix-unqualified-reference-to-nfs-no-hostonly-network-error

Fix wrong reference to Errors::NFSNoHostonlyNetwork
Merge pull request #1447 from uggedal/fix-shell-provision-args-config-validation

Fix shell provisioner's validation of args