Stop relying on internal Functest utils

Stop relying on internal Functest utils

It switches from functest_utils.get_functest_config() to
config.CONF(). download_url is now hosted in sfc as it's unused by
Functest.

The next functions will be considered as part of Functest framework
(then it must be well covered and documented):
  - ft_utils.execute_command_raise()
  - ft_utils.execute_command()
  - ft_utils.get_parameter_from_yaml()

Change-Id: I076c6ced430df8387de8eaeac6f3a94d93201b19
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>