Support setting ohdear_healthcheck_secret via environment variable

Don't forget to add the env var description to README

I've added an option to override ohdear healthcheck secret.

I've added environment variables to override all of the configuration options.

Reviewed. One nit.

I'm usually apprehensive about review without testing but

• changes look clear and non intrusive and will assume you've tested
• I see you're actively working on the module or want to (e.g. ✨ Show most valueable data from ohdear inside drupal Needs work )
• there's not a whole lot I can test without spending more time setting up the integration, than I may be able to justify right now given the above two points...

I've removed code duplication in controller constructor and fix antipattern (in constructor as well).
Also added a log statement out of laziness.

+1 $requestStack

Merged - will be part of release tomorrow.

Automatically closed - issue fixed for 2 weeks with no activity.