Add force SASL PLAIN for LDAP compliance

It was decided that we should default to PLAIN SASL, since thats the only way it works with LDAP users. If someone needs secured auth, go use encryption anyways since SCRAM is just protecting the creds anyways.

There will be a sys property named forceSaslPlain which is true and can be set to false to get the old behavior back.

Also with this to true, we don't need to ask for the list mechs anymore, and save a roundtrip.