Add a remove_from_config utility

* This enables to remove from the udtc config file any registered framework
* Add corresponding tests
Add removal feature to frameworks

* All frameworks have now a --remove option enabling them to uninstall
  their desktop file, icon, projected content and udtc config.
* Add corresponding tests.
Fix tests mocks to be instantiable

* remove() method is abstract, ensure we can continue to register the mock
  frameworks in tests by adding this function.
* Set the args to False when running run_for
Add small tests on calling the remove function
Add 2 reinstall large tests

* This will enable to ensure that recent code change really remove the
  initial installation, even if we change the destination directory.
Add some comments and FIXME
Switched Download Center to requests.
Fixed small and medium tests.
New SSL certificate: this one is valid for hostname 'localhost'.
Introduced 'requests' as a dependency.
Assorted small tweaks.