Add env vars to set HTML, MD and XML report output

What does this MR do?

• adds the following env vars in order to allow easier configuration:
  • DAST_HTML_REPORT
  • DAST_MARKDOWN_REPORT
  • DAST_XML_REPORT
• adds some additional tests
• drops zap from the cli help output

What are the relevant issue numbers?

• partially addresses gitlab-org/gitlab#12652 (closed)
• relates to gitlab-org/gitlab#198459 (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
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer