Disallow anonymous binds in LDAP by default

LDAP 'binds' may be anonymous; that is if you don't specify a password
you are automatically authenticated as the anonymous user. This is not
what we want in Impala, and while many LDAP deployments disable
anonymous binds, many don't. We should disable them by default.

Change-Id: I4be48b244c73568d23ce0ece87bda08368cb4038
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2453
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins