Fix ownership of /etc/init/xvfb.conf

Puppet defaults to the real uid, not the effective uid, for file owners.
Because we run puppet apply through sudo, this means that this file ends
up being owned by the user who ran the deploy.  I was alerted by this
line of output:

    [ci-slave-1] out: Notice: /Stage[main]/Xvfb::Service/File[/etc/init/xvfb.conf]/owner: owner changed 'alexmuller' to 'ppotter'
Merge pull request #153 from alphagov/fix-ownership

Fix ownership of /etc/init/xvfb.conf