auth_to_local should support reading rules from a file

The current handling of zookeeper.security.auth_to_local in KerberosName.java only supports rules given directly as property value.

These rules must therefore be given on the command line and:

• must be escaped properly to avoid shell expansion
• are visible in the ps output

It would be much better to put these rules in a file and pass the file path as the property value. We would then use something like -Dzookeeper.security.auth_to_local=file:/etc/zookeeper/rules.

Note that using the file: prefix allows keeping backward compatibility.