code: def install_custom(self, *args, **kwargs):
          
          if not self.env.manage_custom:
              return
          packager = self.packager
          if packager == APT:
              return self.install_apt(*args, **kwargs)
          elif packager == YUM:
              return self.install_yum(*args, **kwargs)
          else:
              raise NotImplementedError('Unknown packager: %s' % (packager,))
docs: Installs all system packages listed in the appropriate
          <packager>-requirements.txt.