Fix for SSH key arrays in Vagrant 1.4+

In Vagrant 1.4 and greater the SSH key can be an array. This was causing a problem even when it was a single key, because the string was getting printed as ["/path/to/key"], including the brackets and quotes, and hence wasn't working.

This fix comes directly from this commit to the openstack plugin:
https://github.com/cloudbau/vagrant-openstack-plugin/commit/48eac2932fa16ccd5fab2e1d2e0d04047f3be7bd