Use official OpenVPN repositories for latest stable versions (#1323)

## Summary

- Install OpenVPN from official upstream repositories instead of
distribution packages
- Gets the latest stable releases with security fixes and new features
- Properly cleans up repos and GPG keys on uninstall

## Repository sources

| OS | Repository |
|---|---|
| Debian/Ubuntu | `build.openvpn.net/debian/openvpn/stable` |
| CentOS/Oracle/Fedora | Fedora Copr `@OpenVPN/openvpn-release-2.6` |
| Amazon Linux/Arch | Distribution packages (no official repo available)
|

## Changes

- Add `installOpenVPNRepo()` function to configure official repos before
package installation
- Remove duplicate package installations between repo setup and install
functions
- Clean up repos and GPG keys during uninstall
- Standardize `log_success` (`[OK]`) for major milestones only


---

Close https://github.com/angristan/openvpn-install/pull/1294

README.md

@@ -113,6 +113,7 @@ export PASS="1"
 ## Features
 
 - Installs and configures a ready-to-use OpenVPN server
+- Uses [official OpenVPN repositories](https://community.openvpn.net/openvpn/wiki/OpenvpnSoftwareRepos) when possible for the latest stable releases
 - Iptables rules and forwarding managed in a seamless way
 - If needed, the script can cleanly remove OpenVPN, including configuration and iptables rules
 - Customisable encryption settings, enhanced default settings (see [Security and Encryption](#security-and-encryption) below)