Group execution of SQL functional tests

Functional SQL tests (MySQL and PostgreSQL) use the DB installed
backend. To avoid overloading the DB engine, the test cases are
grouped [1] together and executed by the same worker process. That
will ensure those tests are not executed in parallel, interfering
each other.

Classes that are grouped:
- TestModelsMigrationsMysql
- TestModelsMigrationsPsql

[1]https://stestr.readthedocs.io/en/latest/MANUAL.html#group-regex

Related-Bug: #1687027

Change-Id: If04e360c01cdb16e956d16357e2181db55445137
(cherry picked from commit 7326fd622f)

tox.ini

@@ -62,7 +62,7 @@ deps =
   {[testenv:functional]deps}
 commands =
   {toxinidir}/tools/deploy_rootwrap.sh {toxinidir} {envdir}/etc {envdir}/bin
-  stestr run {posargs}
+  stestr run --group_regex=neutron\.tests\.functional\.db\.test_migrations\.(TestModelsMigrationsPsql|TestModelsMigrationsMysql) {posargs}
 
 [testenv:dsvm-fullstack]
 setenv = {[testenv]setenv}