Cannot change the renewal interval

In our company we have a requirement to renew certificates 40 days before they expire. Currently this cannot be configured in resty-auto-ssl. There should be an option to configure this.
As a workaround I had to do two things
- Change the following line in renewal.lua by replacing the 30 with 40 ` if now + (30 * 24 * 60 * 60) < cert["expiry"] then`
- Set RENEW_DAYS="40", because otherwise dehydrated will do nothing

It would be great if we had an option to configure the number of days. The same value could/should be used for the lua code and the dehydrated parameter.

Start with the renewal.lua condition shown in the issue and trace how resty-auto-ssl passes configuration to dehydrated, including the RENEW_DAYS setting. Done means the renewal interval is configurable and the same configured value governs both the Lua check and dehydrated behavior.

Written by the indexing model from the issue text.