Define "integration" and "functional" tests

[adrienverge]

It seems that not everyone agree on "integration" and "functional" definitions. It is important that we use the same naming for every project, and stick to it.

For what it's worth, I used to use these definitions (but I'm OK to change):

• functional: tests that ensure a process (such as repd, vaultd, allocd, datad) behaves correctly when running in some specific environment.
• integration: tests that run all processes to have a whole system (using Docker for instance) and make sure processes interact correctly to provide a global product that is working.

@rachedbenmustapha @DavidPineauScality @MichaelZapataScality @vrancurel