Fix invalid test in cinder data migration script

The extend_start.sh script for cinder-api uses an incorrect test to
check if the MAX_NUMBER variable is set. This leads to a incorrect
cinder-manage command being run if this variable is not set, making the
database migration fail.

This change fixes the test to properly use the MAX_NUMBER variable.

Change-Id: Ie46da709db0de09a9cf641b3c154275282f213fc
Closes-Bug: #1866827
(cherry picked from commit ff42ff2)

Author: gpocentek

docker/cinder/cinder-api/extend_start.sh

@@ -9,7 +9,7 @@ if [[ "${!KOLLA_BOOTSTRAP[@]}" ]]; then
 fi
 
 if [[ "${!KOLLA_OSM[@]}" ]]; then
-    if [[ -z ${MAX_NUMBER} ]]; then
+    if [[ "${!MAX_NUMBER[@]}" ]]; then
         cinder-manage db online_data_migrations --max_count ${MAX_NUMBER}
     else
         cinder-manage db online_data_migrations

releasenotes/notes/bug-1866827-5351ec43486d7f33.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixes the MAX_NUMBER variable usage when running the database online
+    migrations for cinder.