Switch from testr to ostestr

Was seeing py34 (and DSVM) tests failing with an Invocation error, even
though the test passed. Switched to ostestr, as is done in Neutron under
commit 240facf9e67aa5b205679f397107abfe059a869a, and the test passes
(not sure what the root cause is here).

Change-Id: If85987865e0393892c52f319d7cef526c021e0df

tools/ostestr_compat_shim.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+# preserve old behavior of using an arg as a regex when '--' is not present
+case $@ in
+  (*--*) ostestr $@;;
+  ('') ostestr;;
+  (*) ostestr --regex "$@"
+esac

tox.ini

@@ -13,8 +13,8 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 whitelist_externals = sh
 commands =
-  sh tools/pretty_tox.sh '{posargs}'
-# there is also secret magic in pretty_tox.sh which lets you run in a fail only
+  {toxinidir}/tools/ostestr_compat_shim.sh {posargs}
+# there is also secret magic in ostestr which lets you run in a fail only
 # mode. To do this define the TRACE_FAILONLY environmental variable.
 
 [testenv:common-constraints]