Support LDAP authentication without bind credentials

[vsupalov]

A non-stated requirement of #374

As part of work on first LDAP support in #374 it turned out that Druid does not seem to support connecting to an LDAP service without bind credentials.

If this scenario is a priority, we will need to investigate it and maybe contribute a fix to Druid.

Various combinations of "don't specify LDAP bind username and password settings" and "set those settings to an empty string" have been tried, but have not resulted in the desired behaviour.