AutoMTLS does not work with brokering from plugin to host.

[azdagron]

With AutoMTLS=true, brokered connections dialed from the plugin back to the host fail with transport: authentication handshake failed: x509: certificate is valid for localhost, not unused.

Additionally, brokering with AutoMTLS requires the use of broker.AcceptAndServe since the autogenerated TLS configuration is not exported and therefore cannot be used to set up a gRPC server to use with the listener returned from broker.Accept.

[fairclothjm]

Fixed in #193