Allow users to override images prefix

Problem to solve

Some customers are using SAST inside a closed network (refs https://gitlab.com/gitlab-org/gitlab-ee/issues/6603). While the sast image can be hosted in their private network, it will try to download images from GitLab registry once a language/framework is detected. We should allow them to specify a prefix similar to the sast image they are using.

Proposal

Let users specify a prefix for all analyzers.

What does success look like, and how can we measure that?

SAST is working completely with Artificatory installations.

ZD ref: https://gitlab.zendesk.com/agent/tickets/97373

/cc @bikebilly