Remove .testrepository/times.dbm

When we run tox -e py27 command and after that
we run tox -e py35 command, we will meet with
test fail problem like: "No module named gdbm".
This patch adds cleaning up of this file
before running next tests.

Change-Id: I35a20521706b19a3067a5579c9dcf46efd0d55c0

tox.ini

@@ -6,12 +6,14 @@ skipsdist = True
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
+whitelist_externals = rm
 install_command =
     pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
        setuptools-git>=0.4
 commands =
+   rm -f .testrepository/times.dbm
    {toxinidir}/tools/ostestr_compat_shim.sh {posargs}
 
 [testenv:functional]