Fix devstack install error

With the removal of screen support,tail_log is deprecated and removed after Queens.

Change-Id: Ia0dadd6cb403e98c988c99935d32804df9b7ed59

devstack/lib/freezer-api

@@ -186,14 +186,14 @@ function start_freezer_api {
         enable_apache_site freezer-api
         restart_apache_server
 
-        tail_log freezer-api /var/log/$APACHE_NAME/freezer-api.log
+        # tail_log freezer-api /var/log/$APACHE_NAME/freezer-api.log
 
     elif [[ "${FREEZER_API_SERVER_TYPE}" == "apache2" ]]; then
 
         enable_apache_site freezer-api
         restart_apache_server
-        tail_log freezer-api /var/log/$APACHE_NAME/freezer-api.log
-        tail_log freezer-access /var/log/$APACHE_NAME/freezer-api_access.log
+        # tail_log freezer-api /var/log/$APACHE_NAME/freezer-api.log
+        # tail_log freezer-access /var/log/$APACHE_NAME/freezer-api_access.log
     fi
 }
 
@@ -331,4 +331,4 @@ function configure_uwsgi_freezer_api_app {
 }
 
 # Restore xtrace
-$XTRACE
+$XTRACE