Better control in .travis.yml about needed services for each job type

This is a followup / related with MDL-66322, where we started to use the moodle-exttests docker image to run those tests 100% locally.

Also, it will impact any other future service in general (containerised or no), say redis, memcached, unoconv, solr...

The basic idea is that we should launch those services exclusively when the job being run requires them. In this case, it's 99.99% about only starting them when "phpunit" jobs are launched, and keep them out for the rest of jobs (upgrade, grunt, ...).

But in general, any job should have its organized place to start needed services (say grunt tomorrow requires xxxx that is available via docker... or whatever).

That's the idea. Ciao