Run tests sequentially in tempest_slow

Run tests sequentially in tempest_slow

concurrency is set to 2 in the upstream job [1] but it conflicts with
serial also passed.

As it fails once without any other test in parallel, the number of
executors should be decreased [2].

[0] https://799189c1bf6a1aa9c8de-48970bda2a30ee836a70a12e61555969.ssl.cf1.rackcdn.com/694893/1/check/tempest-slow/1bf5851/job-output.txt
[1] http://artifacts.opnfv.org/functest/functest-opnfv-functest-smoke-cntt-hunter-tempest_slow-run-7/tempest_slow/tempest-report.html

Change-Id: I51bc9a584cf4b32a88b7a84d9387e7b8e90281c2
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>