Add single quotes for posargs on jobs

People seem to hate the idea of using tox -epy27 -- foo to use a
test regex so add single quotes around posargs to make it so you can
just do tox -epy27 foo again.

Change-Id: I12c5d2dc78db3bc8954ff420492115ff53c42703

tox.ini

@@ -29,7 +29,7 @@ passenv = OS_DEBUG GENERATE_HASHES
 [testenv:py27]
 commands =
     {[testenv]commands}
-    stestr run {posargs}
+    stestr run '{posargs}'
     env TEST_OSPROFILER=1 stestr run --combine --no-discover 'nova.tests.unit.test_profiler'
 
 [testenv:py35]
@@ -68,7 +68,7 @@ setenv = VIRTUAL_ENV={envdir}
        LANGUAGE=en_US
 commands =
   find . -type f -name "*.pyc" -delete
-  stestr run {posargs}
+  stestr run '{posargs}'
 
 # TODO(gcb) Merge this into [testenv:functional] when functional tests are gating
 # with python 3.5
@@ -91,7 +91,7 @@ setenv = VIRTUAL_ENV={envdir}
        LANGUAGE=en_US
 commands =
   find . -type f -name "*.pyc" -delete
-  stestr run {posargs}
+  stestr run '{posargs}'
 
 [testenv:genconfig]
 commands = oslo-config-generator --config-file=etc/nova/nova-config-generator.conf