Run unittests under the low-constraints job

The low-constraints job should run unittests by the original desgin, we
didn't put the commands about running unittests inside the default
testenv settings, so in fact the low-constraints job did nothing except
creating a virtualenv.

This patch fixes and also move the unittests commands to the default
testenv settings like the other OpenStack projects.

Change-Id: Ia344a14a9baf7574b043fc08332869083da6dc1e
Signed-off-by: Zhao Chao <zhaochao1984@gmail.com>

tox.ini

@@ -19,6 +19,8 @@ deps = -r{toxinidir}/requirements.txt
 commands = find ./trove -type f -name "*.pyc" -delete
            rm -f .testrepository/times.dbm
            rm -f trove_test.sqlite
+           stestr run --serial '{posargs}'
+           stestr slowest
 whitelist_externals = find
                       rm
                       bash
@@ -32,22 +34,6 @@ commands =
     flake8
     doc8 {posargs}
 
-[stestr-base]
-commands = stestr run --serial '{posargs}'
-           stestr slowest
-
-[testenv:py27]
-commands = {[testenv]commands}
-           {[stestr-base]commands}
-
-[testenv:py34]
-commands = {[testenv]commands}
-           {[stestr-base]commands}
-
-[testenv:py35]
-commands = {[testenv]commands}
-           {[stestr-base]commands}
-
 [testenv:apiexamples]
 commands = {envpython} generate_examples.py