change genconfig output-file path

Because original directory 'etc/octavia' does not exist, excuting
'tox -egenconfig' fail with 'No such file or directory'.

Change-Id: I1b26bc1e631d1280dab223b1271362100347fe82

tox.ini

@@ -80,7 +80,9 @@ commands =
 commands = {posargs}
 
 [testenv:genconfig]
+whitelist_externals = mkdir
 commands =
+         mkdir -p etc/octavia
          oslo-config-generator --output-file etc/octavia/octavia.conf.sample \
          --namespace octavia \
          --namespace oslo.db \