Add SAML_SCHEME_OVERRIDE config

[jhult]

getredash/redash#3614

This setting will allow you to override the SAML Auth URL scheme that gets constructed by Flask. This is a useful feature if, for example, you're behind a Proxy Protocol enabled TCP load balancer (AWS ELB that terminates SSL) and your Nginx proxy or similar adds a X-Forwarded-Proto of HTTP even though your Redash URL for SAML auth is HTTPS.

Example use in my-values.yaml:

redash:
  host: https://mysecuredomain.com
  samlSchemeOverride: https

[grugnog]

Thanks!