In anticipation of puppet 4, start trying to deal with puppet 4 things
that can be helpfully predicted by puppet lint plugins. Also fix lint
errors caught by the puppet-lint-empty_string-check gem:
- releasestatus base class: replacing empty string defaults with undef
- releasestatus::site class: replacing empty string defaults with
reasonable default values inspired by system-config's usage
Change-Id: I7ce45ac86da0d7dcdb90deb292973c8cdde0421a