Update tempest_custom in userguide

Update tempest_custom in userguide

It was mostly only used by OVP (Functest leverages regexes).
It's now up-to-date about stevedore.

Functest users are free to overwrite any refstack list (e.g.
/home/opnfv/functest/data/refstack/platform.txt) if they want the same
behavior [1].

[1] https://git.opnfv.org/functest/tree/docker/smoke/platform.txt

Co-Authored-By: Roman Kuzmin <rkuzmin@mirantis.com>
Change-Id: I2b6c3ceda102319bfea810ab2a7bf2d911e20f6b
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>