Add support for overriding st2.conf settings via env vars

[cognifloyd]

This PR's commits were extracted from #6273 where I'm working on getting pants+pytest to run integration tests.

We have upgraded to a version of oslo_config that supports overriding conf vars via environment variables. That support is enabled by default, but it hardcodes the env var prefix OS_. Since oslo_config is designed for use within OpenStack, the OpenStack devs did not have a reason to make that prefix configurable. Using OS_ does not make sense for the StackStorm project, so I changed the prefix to ST2_ in this subclass:

st2/st2common/st2common/config.py

Lines 904 to 908 in 4e8794c

	class St2EnvironmentConfigurationSource(EnvironmentConfigurationSource):
	@staticmethod
	def get_name(group_name, option_name):
	group_name = group_name or "DEFAULT"
	return "ST2_{}__{}".format(group_name.upper(), option_name.upper())

To use this class, we have to update the cfg.CONF global to use our subclass instead of the default. This is done with a helper method:

st2/st2common/st2common/config.py

Lines 911 to 913 in 4e8794c

	def use_st2_env_vars(conf: cfg.ConfigOpts) -> None:
	# Override oslo_config's 'OS_' env var prefix with 'ST2_'.
	conf._env_driver = St2EnvironmentConfigurationSource()

This helper needs to be called just before we tell oslo to parse the config (by calling cfg.CONF(...)), which happens in various parse_args() functions throughout the ST2 codebase, including the primary one here:

st2/st2common/st2common/config.py

Lines 916 to 923 in 4e8794c

	def parse_args(args=None, ignore_errors=False):
	use_st2_env_vars(cfg.CONF)
	register_opts(ignore_errors=ignore_errors)
	cfg.CONF(
	args=args,
	version=VERSION_STRING,
	default_config_files=[DEFAULT_CONFIG_FILE_PATH],
	)

Finally, I made use of this feature to pass settings to Integration test subprocesses. When running the tests via pants+pytest, we have to modify things like the db-name to use a slot number so that tests can run safely in parallel without clobbering other tests' data. That slot number only gets injected in test code (in st2tests.config), so we needed a way to override the conf file on the fly with the updated settings for the test. For example, this test now injects various env vars to ensure the subprocess uses the conf values that the test expects:

st2/st2common/tests/integration/test_service_setup_log_level_filtering.py

Lines 232 to 238 in 4e8794c

	env = env or os.environ.copy()
	env.update(st2tests.config.db_opts_as_env_vars())
	env.update(st2tests.config.mq_opts_as_env_vars())
	env.update(st2tests.config.coord_opts_as_env_vars())
	process = subprocess.Popen(
	cmd,
	env=env,

These env var helpers are defined in st2tests.config to make it as easy as possible for tests to inject the relevant env vars.

Bonus: This will make configuration for docker and kubernetes much more straightforward. We have had requests to make secret vars in particular settable via env vars in addition to the conf file. So, this makes that possible.