Travis CI and Coverity Scan integration

[Dridi Boukelmoune]

Static salutations,

We haven't had a Coverity Scan build since Dec 6th despite having a
Travis cron job responsible for submitting builds, and allegedly
submitting them successfully.

See for example the latest job:

https://travis-ci.org/github/varnishcache/varnish-cache/builds/752636440#L1209-L1210

An yet, nothing showed up.

Our Travis CI configuration currently handles Coverity Scan builds
manually, but there is a Travis CI addon that should spare us a lot of
trouble. I have used it in my cashpack project for a while. When I put
this setup in place, Travis build stages didn't exist and it was really
cumbersome but I experimented with it and ended up with the following
changes:

https://github.com/Dridi/cashpack/compare/8d31c2b...a53aed9?w=1

It's easier to look at individual diffs. In the case of cashpack, it's
not a cron job that triggers static analysis but a dedicated branch.
There's too little traffic on the master branch to justify a cron job.

My key takeaways:

- the Travis CI addon requires less maintenance
- the addon's output is much more detailed
- stages make conditionals simpler

See this build for example:

https://travis-ci.org/github/Dridi/cashpack/builds/752960297

We already have stages in our Travis CI build, is there anyone
objecting to replacing our handrolled integration with the addon?

I will time this question out in one week in the absence of a
convincing objection.

Statically,
Dridi