[coverage] enable coverage in autobuild.py and publish html report to gitlab pages with ci

[joeg at catalyst.net.nz]

Hi team,

I am currently trying to add a new option `--enable-coverage` to
`autobuild.py`,

so we can enable this option and publish the html report to gitlab pages
automatically with gitlab ci.

(Actually we did have coverage report 7 years ago, according to Andrew
Bartlett.)


I already got a decent report here:
https://samba-team.gitlab.io/devel/samba/

The pipeline/branch which generated this report is here:
https://gitlab.com/samba-team/devel/samba/pipelines/59682470


I am still polishing things so the report may not be accurate, but happy
to share this report in advance.

If any one discovers problems from the above report or has fancy ideas,
please let me know.

New tests are welcome to increase the coverage percentage from now on:)


-- 
Joe Guo
joeg at catalyst.net.nz
Catalyst IT