Find all the available bootstraps and return them.

def list_bootstraps(cls):
        '''Find all the available bootstraps and return them.'''
        forbidden_dirs = ('__pycache__', 'common')
        bootstraps_dir = join(dirname(__file__), 'bootstraps')
        for name in listdir(bootstraps_dir):
            if name in forbidden_dirs:
                continue
            filen = join(bootstraps_dir, name)
            if isdir(filen):
                yield name