Since [1], Tempest is a pip installable package, and that prevents zuul_cloner
to work correctly. This change moves away from the existing logic in tox_install,
and adds tempest as an explicit requirement for the api job. This is in fact
the only tox target that needs Tempest to work.
tox_install.sh has become less important now, but cleanup is left as follow
up, to speed up gate salvation.
[1] I25eac915c977ebaedced66ac896c5dd77259d193
Depends-on: I1d1ca087982d7bdda983686775736ed258026aeb
Change-Id: I00d882dde77a687ecb57ec200a34fd96256ad87a
8a6913c534