Optionally force username from LDAP authentication data to be lowercase in Ambari

Optionally force username from LDAP authentication data to be lowercase in Ambari based on LDAP import configuration.

In some cases the username declared in the relevant LDAP object is in all uppercase characters when the local Hadoop cluster expects the username to be all lowercase. As of Ambari 2.4.0, the username specified from the LDAP data is used to override the username known to Ambari. This overwritten data may be in all uppercase characters, potentially breaking local username conventions.

To help this scenario, provide a configuration option to force the username obtained from the LDAP object to be converted to all lowercase character.

For example authentication.ldap.username.forceLowercase.

This optional configuration value is to default to false to maintain current functionality.