The switch from testr to stestr introduced in Change-Id
Icf1f9eb4539666dffb6c9c1651670541523a9f9e missed setting concurrency
jobs to 1 for api and scenario tests, leading to intermittent gate
issues.
This patch also fixes coverage test that got broken in the
aforementioned Change-Id.
Change-Id: I052a9c3b376624f8e9e63faa98f4d09a93bdde4d