software/grafana: fix smtp_verify_ssl option handling

This software uses xml for parameters serialisation, so we can not use
boolean type. As a result, smtp_verify_ssl was always "true" and disabling ssl
verification was not working.

When accessing the smtp server from localhost, grafana attempt STARTLS and
verifies certificate, so scenarios of using localhost:25 as smtp server usually
needed smtp_verify_ssl="false".