[vstest]: change the redis mount point when --dvsname is provided. (#663)

unify the redis mountpoint to /var/run/redis-vs/{base-container-name}

user can use --keeptb to create a testbed and later use --dvsname
to rerun test on the testbed. In this case, we need to use same redis
mount point

Author: lguohan

tests/README.md

@@ -47,7 +47,7 @@ persists.
     ```
     docker run --privileged -id --name sw debian bash
     sudo ./create_vnet.sh sw
-    docker run --privileged -v /var/run/redis-vs:/var/run/redis --network container:sw -d --name vs docker-sonic-vs
+    docker run --privileged -v /var/run/redis-vs/sw:/var/run/redis --network container:sw -d --name vs docker-sonic-vs
     ```
 
 - Run test using the existing vs container

tests/conftest.py

@@ -176,7 +176,7 @@ def __init__(self, name=None, keeptb=False):
                 server = VirtualServer(ctn_sw_name, self.ctn_sw_pid, i)
                 self.servers.append(server)
 
-            self.mount = "/var/run/redis-vs/"
+            self.mount = "/var/run/redis-vs/{}".format(ctn_sw_name)
 
             self.restart()
         else: