KNOX-3422: Ldap Proxy used in-memory bind user (#1357)

* KNOX-3422: Ldap Proxy used in-memory bind user

The bind user created from the gateway configuration is now created
in-memory instead of being added to the embedded ldap server. This
provides the benefit of being able to rotate the user just by changing
the configuration. Previously, since the configured user is not tracked,
there wasn't a way to automatically rotate the user.

* throw LdapAuthenticationException