OPENDJ-2516: improve configuration validation for tamper proof logging

This is a partial fix for OPENDJ-2516 since it traps most configuration
mistakes:

* verify that the key store file and pin properties are specified when
tamper evident logging is enabled
* verify that the key store file and pin file are readable and not
empty.

This change does not validate the content of the keystore which should
really be the responsibility of CAUD. However, the error reporting in
this case does not seem to be working very well, hence this change is
just a partial fix.