More Kerberos goodies (keytab support + external config)

This adds support for authenticating with a keytab file. I also added the ability to use an external JAAS config so settings can be entirely overwritten by providing a configuration for "pentaho".

This does not include auto-refreshing of keytab tickets. That work is left to do.

Keytab support is plumbed through via transformation properties today. The 2 properties you must set are:

`PENTAHO_JAAS_AUTH_MODE=KERBEROS_KEYTAB`
`PENTAHO_JAAS_KEYTAB_FILE=/home/user/my.keytab`

More details can be found in `KettleKerberosHelper`.