Use 'tempest run --slowest' in integrated-compute (again)

Change I53b0fa6dcf077191565e2aaf3f08a24b3238a34b added the --slowest
option to the tempest run command for scenario and serial tests but
missed adding it when running api tests.

The --slowest option will show a list of the slowest tests after the
tests finish running.

Change-Id: Ia5a995121748a2c244f060e6ce4b395213bb196e
diff --git a/tox.ini b/tox.ini
index fcdf6ff..e3c8fcf 100644
--- a/tox.ini
+++ b/tox.ini
@@ -197,7 +197,7 @@
 # tests listed in exclude-list file:
 commands =
     find . -type f -name "*.pyc" -delete
-    tempest run --regex {[testenv:integrated-compute]regex1} --exclude-list ./tools/tempest-integrated-gate-compute-exclude-list.txt {posargs}
+    tempest run --slowest --regex {[testenv:integrated-compute]regex1} --exclude-list ./tools/tempest-integrated-gate-compute-exclude-list.txt {posargs}
     tempest run --combine --serial --slowest --regex {[testenv:integrated-compute]regex2} --exclude-list ./tools/tempest-integrated-gate-compute-exclude-list.txt {posargs}
 
 [testenv:integrated-placement]