Fix broken node labels when using with Vagrant 2.0 (#816)

A broking change has been introduced in Vagrant 2.0 with this commit:
https://github.com/hashicorp/vagrant/commit/ac75e409a3470897d56a0841a575e981d60e2e3d
The change puts additional quotes around the node labels.

As a fix, use different quoting mechanism for Vagrant >=2.0