Settle on how to treat openvpn options

[nkakouros]

The way openvpn config options are supported is a bit messy. It is a long list of possible values with no real cohesion. Many PRs have to do with adding this or that option. There are two ways to move forward:

1. Support directly options that are 100% essential (by essential I mean options without which the config file will be invalid) and options that need special handling. The rest will be supported only via openvpn_server_options.

2. Support all options via some logic. Instead of including every option in defaults/main.yml, the approach could be like in this or this role, ie have some kind of logic that checks/generates options either via some role tasks or via a shell script.

Though option 2 would feel nice to have in terms of completeness, openvpn is quite complex and option 1 seems the more sane approach.

Waiting for feedback by anyone interested.