Fix: Split concrete and abstract dependencies

Fix: Split concrete and abstract dependencies

It adds 2 upper-constraints to pin all the dependencies.
It modifies a little bit the official upper-constraints.txt from
OpenStack due to Sphinx (4.2.0 as asked by lfdocs-conf vs 3.5.2) and
due to setuptools (58.1.0 as used in ONAP gates vs 54.1.1).

In a long run, upper-constraints.os.txt should be removed once LF/ONAP
is synced with OpenStack. For it's part, upper-constraints.lf.txt
should be centralized for all ONAP/LF projects and then removed from
this particular project.

It also adds test-requirements.txt to list all test requirements (see
tox.ini) and lists sphinx contrib modules in requirements.txt which are
selected by the default configs. Otherwise the project must falsy lists
them as deps to complete the missing lfdocs-conf requirements.

Issue: RELENG-3968

Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: I38057cdd0f3553d64c8a682faf0d304b680fcb96