Add gemnasium-maven as a child pipeline

What does this MR do?

• Add a child pipeline to build, test, and push the gemnasium-maven images.
• Fix: Remove gemnasium-maven and gemnasium-python from the dependencies of the Go project.
• Move security scans to post-image stage. See discussion in !317 (merged)
• Align Python builders w/ Maven builders, so that they share the same Go interface.
• Remove qa/fixtures before running security scans.

What are the relevant issue numbers?

gitlab-org/gitlab#198361 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer