compare-puppet-catalogs: specifiy hiera_config

Funnily enough, while puppet's confdir setting is valid for
overriding _every_other_ puppet default, it's not the case with hiera's
configuration, which has its own config switch and is not affected by confdir.

Change-Id: I7299dd3d17cd38cecee7cf09fbfeb12334567c88
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>