Added `openssl_verify_mode` parameter for action_mailer.

This adds the parameter `openssl_verify_mode` to the action_mailer config, so you can override the verification mode from your environment settings, e.g. from docker.

With this patch, simply adding:

    env:
      DISCOURSE_SMTP_OPENSSL_VERIFY_MODE: 'none'

to your docker config will disable certificate validation, for example if you are running a local relay server.

See also [this thread][1] for discussion.

[1]: https://meta.discourse.org/t/provide-more-settings-to-be-overridden-in-globalsetting/11560