Fix directory for volume/snapshot boot pattern

When using full image of ubuntu tmp directory is cleared on boot
so using other directory which is not cleaned.

Related-Prod: https://mirantis.jira.com/browse/PRODX-47216
Change-Id: I3d76d2ab4fcda6e08a840382657b966732fdb11c
(cherry picked from commit 2f19a48ea404ad2405671e21bbda3ff8520965d0)
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index 28127c9..24150c7 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -1264,10 +1264,10 @@
                                             server=server,
                                             username=username)
 
-        # Default the directory in which to write the timestamp file to /tmp
+        # Default the directory in which to write the timestamp file to /root
         # and only use the mount_path as the target directory if we mounted
         # dev_name to mount_path.
-        target_dir = CONF.scenario.target_dir
+        target_dir = '/root'
         if dev_name is not None:
             mount_path = os.path.join(mount_path, dev_name)
             ssh_client.make_fs(dev_name, fs=fs)
@@ -1303,10 +1303,10 @@
                                             server=server,
                                             username=username)
 
-        # Default the directory from which to read the timestamp file to /tmp
+        # Default the directory from which to read the timestamp file to /root
         # and only use the mount_path as the target directory if we mounted
         # dev_name to mount_path.
-        target_dir = CONF.scenario.target_dir
+        target_dir = '/root'
         if dev_name is not None:
             mount_path = os.path.join(mount_path, dev_name)
             ssh_client.mkdir(mount_path)