Extend default setenv instead of replacing it in tox.ini

Extend default setenv instead of replacing it in tox.ini

Some tox jobs[1] define their own setenv without extending/referencing
default setenv, it disallows to define environment variables shared by
all jobs. This change updates previous jobs[1] in order to extend
instead of replacing default setenv and enable global environement
variables (used in daughter change).

One remark, this change sets VIRTUAL_ENV environment variable in updated
jobs[1] but it has no effect on them.

[1] (dsvm-)functional, (dsvm-)fullstack and api jobs

Closes-Bug: #1468059
Change-Id: I99184f7375571fb8569a24ba04ae267108f5da08