Updating the ubuntu/debian apt install process

[bctrainers]

Greetings, per maintainer request, I am opening up an issue prior to pushing a pull request. With my fork, I have updated the Debian/ubuntu initial installation process with the following changes:

• Migrate from apt-key to the gpg process - deprecation warnings.
• Utilize the official OpenVPN repo for the OpenVPN package for all Debian and ubuntu latest releases, and not hard coded to xenial.
• Use lsb_release -cs to set the apt repo version to use.
• Use keyring system for openvpn.list openvpn package.
• Utilize package pinning for OpenVPN.
• Realign apt package installation process.
• Added a gpg sanity check on the imported gpg key. Run manually or uncomment to run on installation. (Needs a better method)

https://github.com/bctrainers/openvpn-install/commit/1a5cc98284795a05cbd8def6365b80a29123502b