Accept an external_links list in the service configuration dictionary to create links to containers outside of the project

Signed-off-by: Jason Bernardino Alonso <jalonso@lumino.so>
Allow external_links config values to have link_names

Signed-off-by: Jason Bernardino Alonso <jalonso@lumino.so>
Fix indentation for flake8

Signed-off-by: Jason Bernardino Alonso <jalonso@luminoso.com>