Add state_path config option

Some tests run and mount files to docker containers, like
scenario dynamic routing. Since we are running tempest in
container the path to child container should be present on
the host (docker starting container from the system).
Right now path is hardcoded to /tmp/ directory. This config
option is needed for that case.

Related-Prod: PRODX-31417
Change-Id: I9d9772c526a5d383ef14d0c9247b7ecdc63957fc
diff --git a/tempest/config.py b/tempest/config.py
index e556895..bc7cb45 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -1408,6 +1408,10 @@
                 help="The boolean flag to specify the type of environment. "
                      "Skip tests that cannot be run in production. "
                      "For example: create/delete TLDs in Designate tests."),
+    cfg.StrOpt('state_path',
+               help="The top-level directory for maintaining Tempest state. "
+                    "For example store configuration files mounted do docker "
+                    "containers."),
 ]
 
 _opts = [