Previously, this didn't actually do anything because log_level
was not an established config value for the python agent.

As of elastic/apm-agent-python#946 this is
now a setting in python and it was causing tests to fail due to the
way python logging works. This PR cleans up all of those references
and will make tests pass.

* Remove ELASTIC_APM_LOG_LEVEL from python stuff

Previously, this didn't actually do anything because log_level
was not an established config value for the python agent.

As of elastic/apm-agent-python#946 this is
now a setting in python and it was causing tests to fail due to the
way python logging works. This PR cleans up all of those references
and will make tests pass.

* Remove default for --apm-log-level

All uses of this setting use `self.options.get` with a default,
so we can remove the default in `argparse`. This allows the python
agent to only use this setting if it's explicitly set, which is better
for how the python agent's `log_level` works.

* Fix lint

* Remove unused arg from conf.setup_logging

* Add ValidValuesValidator

* Add tests for ValidValuesValidator

* Add support for callbacks_on_default

Also refactored one of the `call_callbacks` methods to
`call_all_callbacks` for clarity.

* Add log_level config with stubbed callback

* Don't run callbacks on copy operations

* Move callbacks to the end of any update() operation

This allows for callbacks to be config aware, and makes the full
config available to callback functions.

* Add log_file/size and implement logging setup callback

* Fix docs link

* Add changelog

* Audit logger calls

* Don't mess with global logging library

* Add tests

* Fix missing arg for ConfigurationError

* Import logging.handlers

* Implement review suggestions

* Fix windows tests

* Don't set log_level by default

* Better handling for callbacks if default is None

* With new default handling, don't need to check for new_value

* Allow `warn` in addition to `warning` for log_level