[devtools] set .config in setUpClass instead of using a cache in a classproperty
This is much clearer and avoid problem with py.test introspection that triggered
creation and caching of the configuration before actual test execution. This
caused pb in unittest_postgres.PostgresStatementTimeoutTC where configuration
was built before module's setup, hence source configuration was missing
information about where to find the pg cluster.
bgcolor = '#000000'
stylesheets = ['%s/cubicweb.css' % datadir_url]
logo = '%s/logo.png' % datadir_url
lazy = lazystr('%(bgcolor)s')