feat: allow the configuration of any opcache variable

[jpiron]

This removes the need to update the role to support new opcache variables.

[shakalandy]

Cool, this would make my PR #416 obsolete, but keep the default, right?

[jpiron]

Yes, except for the php_realpath_cache_ttl variable.
And indeed it keeps all the defaults as long as you don't define the php_opcache_* variables.
I updated the README to make it more clear.

[shakalandy]

@geerlingguy could we get this one merged? I would adapt #416 to include php_realpath_cache_ttl only - thanks!