Respect auth_to_local rules from hdfs configs (core-site.xml) for LDAP authentication too

IMPALA-2660 added principal mapping for Kerberos authentication but not for LDAP authentication.
Both Kerberos and LDAP are using Active Directory as a backend to store principals (in many cases), so it's naturally to think this should work for LDAP too
http://blog.cloudera.com/blog/2014/10/new-in-cdh-5-2-impala-authentication-with-ldap-and-kerberos/
Fact that IMPALA-2660 works only for Kerberos and not for LDAP principals, breaks authentication in our organization.