Update dev security workflow template

What does this MR do?

This MR updates the security process so instead of a single RM getting assigned to the security RMs, they will look for "Merge into Security" MRs, ready to be merged into their respective security branch.

Link to discussion: https://gitlab.slack.com/archives/CD5LXKH9U/p1540284153000100?thread_ts=1540283424.000100&cid=CD5LXKH9U

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides