stop using 'ERP5Site_getUpgraderSignature(key)' but use 'ERP5Site_getUpgraderSignature().get(key, default_value)' instead for better understandability of codes.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43269 20353a03-c40f-0410-a6d1-a30d3c3de9de