Fix the failure of integration-test

Since urfave/cli v1.22.3, string flag no longer persists the default value if the flag is explicitly initialized.
Please refer to urfave/cli#981.

Signed-off-by: Derek Su <derek.su@suse.com>

integration/common/core.py

@@ -110,7 +110,7 @@ def create_engine_process(client, name=ENGINE_NAME,
                           binary=LONGHORN_BINARY,
                           listen="", listen_ip="localhost",
                           size=SIZE, frontend=FRONTEND_TGT_BLOCKDEV,
-                          replicas=[], backends=["file"],
+                          replicas=[], backends=["tcp", "file"],
                           disable_revision_counter=False):
     args = ["controller", volume_name]
     if frontend != "":