Parsing of plugin-specific envvars will fail when facing a plugin name with a dash

[happz]

Like verify-installation: TMT_PLUGIN_PREPARE_VERIFY_INSTALLATION_VERIFY would be split into prepare step, verify plugin, and installation-verify key, instead of prepare, verify-installation, and verify. We might need to extend plugin API to ask plugins to claim variables, or provide their "identification" substring for envvars, and use it instead of the "plugin name is just a single word" rule.