ovn migration: Fix check for stack name

[1] Updated the migration script to check for config-download
directory instead of stack, but missed update the Error
message.

check_stack function is renamed to check_source_inventory
as now it only checks for source inventory instead of heat
stack.
If source inventory file doesn't exist the script
will report Error message and exit.

This is follow up of [1].

[1] https://review.opendev.org/c/openstack/neutron/+/834925

Related-Bug: #1966099
Change-Id: I2416fba50fc495da4d59a3f335f33d831ca6e91d
(cherry picked from commit ccf8e71efa)

tools/ovn_migration/tripleo_environment/ovn_migration.sh

@@ -145,6 +145,7 @@ EOF
 }
 
 # Generate the inventory file for ansible migration playbook.
+# It uses tripleo-ansible-inventory.yaml which was used during deployment as source inventory
 generate_ansible_inventory_file() {
     local dhcp_nodes
     local inventory_file="$OOO_WORKDIR/$STACK_NAME/config-download/$STACK_NAME/tripleo-ansible-inventory.yaml"
@@ -222,11 +223,12 @@ EOF
     echo "Please review the file before running the next command - setup-mtu-t1"
 }
 
-# Check if the stack exists
-function check_stack {
-    if [ ! -d $OOO_WORKDIR/$STACK_NAME/config-download/$STACK_NAME ]; then
-        echo "ERROR: STACK_NAME=${STACK_NAME} does not exist. Please provide the stack name or its ID "
-        echo "       via STACK_NAME environment variable."
+# Check if source inventory exists
+function check_source_inventory {
+    local inventory_file="$OOO_WORKDIR/$STACK_NAME/config-download/$STACK_NAME/tripleo-ansible-inventory.yaml"
+    if [ ! -f $inventory_file ]; then
+        echo "ERROR: Source Inventory File ${inventory_file} does not exist. Please provide the Stack Name and TripleO Workdir"
+        echo "       via STACK_NAME and OOO_WORKDIR environment variables."
         exit 1
     fi
 }
@@ -349,7 +351,7 @@ command=$1
 ret_val=0
 case $command in
     generate-inventory)
-        check_stack
+        check_source_inventory
         oc_check_public_network
         generate_ansible_inventory_file
         generate_ansible_config_file