Ensure future parser compatibility (quote string)

Since Puppet 3.5.1 the future parser no longer matches upper case words
in a selector statement as they are treated as a custom resource type.

See https://tickets.puppetlabs.com/browse/PUP-2800?focusedCommentId=76016&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-76016 for details.