Openvpn: Migrate to Elliptic Curve Cryptography

• Time measurement on i7 processor
  An RSA 4096 DH key generation takes anywhere from 1 minute to 1 hour.
  ECC takes around 5 seconds consistently.
• Upgrade process
  • Provide option in UI to upgrade
  • Explain implications
  • Notification on upgrade
• Run setup on OlinuXino Lime 2 to measure time taken
  Took 1 minute 47 seconds
• Eliminate the setup step currently required [0/2]
  • Change functional tests
  • Manual page needs updates
• Get rid of unused legacy code
• Generating a new profile should work for old OpenVPN setups still using RSA
• New installations should use ECDH by default. They should not see the upgrade option
• A manual upgrade option should be provided in the configuration page with the consequences clearly described
• Manual test to verify that VPN functionality still works
• Determine the correct curve to be used

Fixes #1514 (closed)