Add 'rm -f .testrepository/times.dbm' command in testenv

Running py2* post py3* tests results in error. Add
'rm -f .testrepository/times.dbm' command in testenv to
resolve this.

Change-Id: I17f872b6169317de7c541a2ed77344ab1c313f94
Closes-Bug: #1229445

tox.ini

@@ -18,6 +18,7 @@ deps = -r{toxinidir}/requirements.txt
 passenv = TEMPEST_* OS_TEST_*
 commands =
   find . -type f -name "*.py[c|o]" -delete
+  rm -f .testrepository/times.dbm
   bash tools/pretty_tox.sh '{posargs}'
 
 [testenv:debug]