Update check signature

[coignetp]

What does this PR do?

Update check signature from check(self, instance) to check(self, _)
Now throw a ConfigurationError in the constructor if the required stats_url option is not in the configuration file, instead of a KeyError in the check method

Motivation

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #8258 (0a10c01) into master (af8a8ff) will increase coverage by 5.22%.
The diff coverage is 68.00%.

Impacted Files	Coverage Δ
envoy/datadog_checks/envoy/envoy.py	71.42% <57.89%> (-4.15%)	⬇️
envoy/tests/test_envoy.py	100.00% <100.00%> (ø)
php_fpm/tests/test_unit.py
...s_wildfly/datadog_checks/jboss_wildfly/__init__.py
...cheduler/datadog_checks/kube_scheduler/__init__.py
hivemq/tests/conftest.py
nagios/tests/test_unit.py
proxysql/datadog_checks/proxysql/ssl_utils.py
kubelet/datadog_checks/kubelet/prometheus.py
... and 801 more