[eduVPN-deploy] Support for Red Hat Enterprise Linux 7

[François Kooman]

Hi,

I pushed some updates to the deploy_centos.sh script to also support Red
Hat Enterprise Linux. This mainly consisted of enabling the optional and
extras subscription and installing the epel-release package manually.

This was tested with a clean install of the recently released Red Hat
Enterprise Linux 7.5 Server ISO and it works flawlessly ;-)

The only requirement is that the machine is subscribed to Red Hat and
can receive updates.

Let me know if you have any questions!

Cheers,
François