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Avoid incorrectly prompting user to sign certificate

On controller, puppet agent can find an error parsing nova.conf:

    Error: /Stage[main]/Openstack::Controller/Resources[nova_config]: Failed to generate additional resources using 'generate': Could not parse line "[DEFAULT]" at /etc/nova/nova.conf:3

It will return 2, but this is harmless. The node will be correctly configured.

See puppetlabs/puppetlabs-nova#161