Subtree scope is not set when retrieving display names from LDAP user store

[vihanga-liyanage]

Moved from https://wso2.org/jira/browse/IDENTITY-7266

Please check here [1] inside "doGetDisplayNamesForInternalRole" method. We are not setting the scope as subtree. It is needed to set it search inside multiple tree

By default it is set to one level scope [2]

[1] https://github.com/wso2/carbon-kernel/blob/4.4.x/core/org.wso2.carbon.user.core/src/main/java/org/wso2/carbon/user/core/ldap/ReadOnlyLDAPUserStoreManager.java#L1039

[2] https://docs.oracle.com/javase/8/docs/api/javax/naming/directory/SearchControls.html