Enable keyfromlabel and enginepkcs11 systemtests for pkcs11-provider

• Simplify configuration management by deducing SoftHSM module path from openssl config
• Determine the engine flag (-E) value from openssl config
• Drop unused/unneeded environment variables

cc @ondrej @marka @matthijs

This generalizes the system tests to work with engines and providers, and simplifies the needed environment variables. Once this, the other few MRs, and the fixed pkcs11-provider (and image is updated), we can finally enable the provider based pkcs11 tests.