Fixes onboarding_extensions_map issue

[PhillSimonds]

This PR addresses the following issues:

• The onboarding_extensions_map setting (specified in netbox_onboarding/init.py
  or overridden in configuration.py) is used to map napalm driver names to a custom
  class which extends the driver, allowing extensibility. Currently, when a mapping
  doesn't exist for a napalm driver, the NetdevKeepr class's get_onboarding_facts()
  method fails. This causes the rq-worker to be unable to run the onboard_device()
  function to onboard a device. The changes in this commit fix the issue.

• Logging is not instantiated in the same way from file to file, and thus log messages
  don't come through (at all in some cases) or consistently (in others). This PR standardizes
  logging such that logs are consistently displayed while troubleshooting

• Currently, the template_content.py DeviceContent class returns None if onboarding is
  not enabled for an OnboardingDevice object. Likewise, if no OnboardingDevice object
  exists, the template continues trying to access attributes for an OnboardingDevice object.
  In the first case, template rendering will fail as an empty string is needed in order to insert
  nothing into the rendered HTML template presented to the user. In the second case, an
  AttributeError is raised as you can not access attributes of a NoneType object. This PR returns
  empty strings in both cases to ensure the HTML renders correctly.

[dgarros]

Also @PhillSimonds please can check if we need to fix the logger on other files, would be good to do all at once

[glennmatthews]

Since you're fixing the logging, consider enabling logging for rq.worker in development/base_configuration.py. Something like:

LOGGING = {
    "version": 1,
    "disable_existing_loggers": False,
    "formatters": {"rq_console": {"format": "%(asctime)s %(message)s", "datefmt": "%H:%M:%S",},},
    "handlers": {
        "rq_console": {
            "level": "DEBUG",
            "class": "rq.utils.ColorizingStreamHandler",
            "formatter": "rq_console",
            "exclude": ["%(asctime)s"],
        },
    },
    "loggers": {"rq.worker": {"handlers": ["rq_console"], "level": "DEBUG",}},
}

[PhillSimonds]

Cool! I'll throw something in!

[dgarros]

@glennmatthews @mzbroch Anything else missing before we can merge this one ? it would be useful to get a beta2 ASAP with these fixes.
My recommendation would be to add more unit tests in a separate PR.

[glennmatthews]

Looks good to me, will keep an eye out for a follow-on PR with unit test improvements. :-)

[mzbroch]

LGTM