Support TLS client-cert authentication for Alphas configured with TLS cert.

[danielmai]

Some users running TLS-enabled clusters with client-auth want to still be able to connect to the cluster with Ratel.

Ratel currently supports server-auth, but not client-auth.

[danielmai]

Ah, this is an existing issue reported at https://github.com/dgraph-io/dgraph/issues/2303

[paulftw]

implemented in master

[paulftw]

I thought this is for ACL. Reopening

[paulftw]

Ratel + TLS setup is documented here: https://docs.dgraph.io/deploy/#using-ratel-ui-with-client-authentication