XD-2121 Add LDAP authentication support

* Establish an xd.security.authentication.ldap configuration root
* Support for LDAP bind authentication with user direct bind and user search
* The intent is for various authentication managers to live in the `org.springframework.xd.dirt.server.security` package, be scanned and activated depending on the configuration properties
* Group search support
* Tests for bind and search-and-bind configuration scenarios

NOTES:

1. Group management, while supported, will have no effect on security, as roles re anot applicable ATM.
2. Password comparison support should make the subject of another JIRA/PR as it is a common feature across various configuration mechanisms.
Can't that `LdapAuthenticationConfiguration` be place in this `@Import` rathen than with `@ComponentScan`? 
Or does that `@ConditionalOnProperty("xd.security.authentication.ldap.enabled")` produce some side effect?