Show section name in CI logs collapsible region

Problem to solve

Currently, in CI logs, it looks like the collapsible regions are placed randomly:

(screenshot of https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/233462571)

Screen_Shot_2019-06-18_at_8.51.59_AM

This is also quite confusing using the new JSON job logs with Docker executor

Screen_Shot_2019-10-22_at_10.40.12

Proposal

For each start of the section, the Runner should write an informative line of what is going on to make it friendlier to the user.

Edited by Steve Xuereb