OpenVPN CentOS7 Edits

I've made a pull request with some updates to reference the correct Diffie hellman parameters in the server.conf file, and correct the location for the sample-config-files directory.  Aside from these edits, this guide passed our technical review.

There are some other edits listed in this PR, but if you pull against the linode/docs master prior to merging, they should be resolved.