Add option for chain.pem file for custom SSL

[kirtangajjar]

Currently, for custom SSL, we only ask for two files from the user as input .crt and .pem file.

We should also ask for an optional .chain.pem file which is needed for OCSP stapling. Custom SSL should run without this file, however if the user has included it, we should keep this file at /opt/easyengine/certs/name.chain.pem. Nginx proxy will then automatically add OCSP stapling config for that site.