Fix flake8 errors in unittest_cwconfig

[config] Test loading of ccplugin modules to ensure ccplugin modules of cube as package aren't loaded twice, once as `cubicweb_<cube>.ccplugin`, once as `cubes.<cube>.ccplugin`. This test used to fail until fe995d56c949 has been backout. It requires calling `cleanup_sys_module` in `CubicWebConfigurationTC.tearDown` to remove any cube's submodule loaded during tests, so content of `sys.modules` may be tested.

[test] Extract out method-which-is-not-a-method mocking iter_entry_points to ease reuse and improve readability.

[cwconfig] Add a docstring on available_cubes method a renaming could be better at some point, but that's a start.

Backed out changeset fe995d56c949 available_cubes should not strip cubicweb_prefix but return actual package name for cube as package, since its output is also used to e.g. get ccplugin or site_cubicweb module name. Original aim of this set was to fix output of the "cubicweb-ctl list" command. This will be done by an alternate implementation in a later cset.

[hooks] Drop "logstats" hook It is now useless as its looping task would not run on a web instance because respective repository has no scheduler.

[hooks] Do not register "logstats" if repository has no scheduler

[server] Introduce an `has_scheduler` method on Repository This is to be used by client application to determine if looping tasks may be registered in the current process. By checking this, one will avoid the warning in looping_task method when the repository has no scheduler.

[server] Warn instead of failing when a looping task is registered and repo has no scheduler We should provide a way for client code to detect if they should register the looping task or not. See next patch for that.

[server] Exit quickly when a looping task is registered in maintenance mode In such cases the repository will not have a scheduler on purpose because the repository will not be kept running and will quickly shutdown after migration so that it's undesirable to have looping tasks being executed.