Reduce the fullstack concurrency to 2

In order to mitigate the OOM problems in the CI, this patch reduces
the fullstack concurrency to 2. That should reduce the memory
consumption and will avoid those nasty OOM exceptions.

Closes-Bug: #1938455
Change-Id: I4de4b089e615353b7fde82bd54e0abefa244c8ab

tox.ini

@@ -80,7 +80,7 @@ deps =
 commands =
   {toxinidir}/tools/generate_dhclient_script_for_fullstack.sh {envdir}
   {toxinidir}/tools/deploy_rootwrap.sh {toxinidir} {envdir}/etc {envdir}/bin
-  stestr run --concurrency 3 {posargs}
+  stestr run --concurrency 2 {posargs}
 
 [testenv:dsvm-fullstack-gate]
 setenv = {[testenv:dsvm-fullstack]setenv}
@@ -88,7 +88,7 @@ deps = {[testenv:dsvm-fullstack]deps}
 commands =
   {toxinidir}/tools/generate_dhclient_script_for_fullstack.sh {envdir}
   {toxinidir}/tools/deploy_rootwrap.sh {toxinidir} {envdir}/etc {envdir}/bin
-  stestr run --concurrency 3 --black-regex neutron.tests.fullstack.test_securitygroup.TestSecurityGroupsSameNetwork.test_securitygroup {posargs}
+  stestr run --concurrency 2 --black-regex neutron.tests.fullstack.test_securitygroup.TestSecurityGroupsSameNetwork.test_securitygroup {posargs}
   stestr run --combine --concurrency 1 neutron.tests.fullstack.test_securitygroup.TestSecurityGroupsSameNetwork.test_securitygroup {posargs}
 
 [testenv:releasenotes]