Fail to start if vulnerability provider is not explicitly configured

[dspalmer99]

Engine should fail to start if vulnerability provider is not explicitly configured. This change is needed to prevent users starting the service via docker-compose, and upgrading to 3.2, from automatically changing their instance to the v2 provider. This change will require them to specifically configure which provider they want to use. The system will fail to start otherwise.

For this change, code needs to change in the following locations to not default to a provider.

• https://github.com/anchore/anchore-engine/blob/master/anchore_engine/services/policy_engine/engine/vulns/providers.py#L1619
• https://github.com/anchore/anchore-engine/blob/master/Dockerfile#L128

Testing machinery also needs to change here:

• https://github.com/anchore/anchore-engine/blob/a638f9753eaa8ce5c5dd5cbf890e6b20ab839790/Makefile#L137-L137 - add to ANCHORE_VULNERABILITIES_PROVIDER="grype"