User configuration option for enabling/disabling Docker Scout

[nightlark]

Users should be able to control whether or not the Docker Scout portion of the docker_image plugin attempts to run. This should be a configuration option that can be set using the surfactant config subcommand. I could see it either being called enable_docker_scout or disable_docker_scout, not really any preference for which one is nicer or more intuitive (but the default value when left unspecified can probably be true).

from surfactant.configmanager import ConfigManager
_config_manager = ConfigManager()
_enable_docker_scout = __config_manager.get("docker", "enable_docker_scout", True)

When the config option is set, it should short-circuit the check_docker_scout_installed and run_docker_scout functions, return immediately and for the latter return {} or None. The values in the class for self.disable_docker_scout = True are essentially locked to self.docker_scout_installed = False as a result of the former check being disabled.

The docker_image plugin itself should still try to run -- the new feature added in #247 will eventually be added to it.

We should also make sure to document the new configuration option (there should be a list with a few macho options elsewhere in the docs/README already. Perhaps even add a bit extra to the Docker Scout not installed warning saying "you can also get rid of this error and skip the check for docker scout by running surfactant config docker.disable_docker_scout true".

[willis89pr]

#326