PostgresConfig should error on unknown SSL_MODES

[ikonst]

PostgresConfig should error on unknown SSL_MODES rather than leaving ssl_mode unset, as it does here:

integrations-core/postgres/datadog_checks/postgres/config.py

Lines 66 to 68 in 3f21b7b

	ssl = instance.get('ssl', "allow")
	if ssl in SSL_MODES:
	self.ssl_mode = ssl

Otherwise it fails down the line with a more cryptic error. For example, I had my ssl parameter set to "true"`, which resulted in

{
  "message": "'PostgresConfig' object has no attribute 'ssl_mode'",
  "traceback": "Traceback (most recent call last):\n  File \"/app/.apt/opt/datadog-agent/embedded/lib/python3.9/site-packages/datadog_checks/base/checks/base.py\", line 1210, in run\n    initialization()\n  File \"/app/.apt/opt/datadog-agent/embedded/lib/python3.9/site-packages/datadog_checks/postgres/postgres.py\", line 740, in _connect\n    with self.db():\n  File \"/app/.apt/opt/datadog-agent/embedded/lib/python3.9/contextlib.py\", line 119, in __enter__\n    return next(self.gen)\n  File \"/app/.apt/opt/datadog-agent/embedded/lib/python3.9/site-packages/datadog_checks/postgres/postgres.py\", line 200, in db\n    self._db = self._new_connection(self._config.dbname)\n  File \"/app/.apt/opt/datadog-agent/embedded/lib/python3.9/site-packages/datadog_checks/postgres/postgres.py\", line 714, in _new_connection\n    'sslmode': self._config.ssl_mode,\nAttributeError: 'PostgresConfig' object has no attribute 'ssl_mode'\n"
}