Fix plufin_conf label

In recent puppet version, @config_label can also be nil instead of
:undef, in puppet templates.
Add regression test for issue #13 / pull request #18
Merge branch 'fix_plugin_template' of github.com:roidelapluie/ssm-munin into roidelapluie-fix_plugin_template
Merge branch 'roidelapluie-fix_plugin_template'