Load DATABASE_USER from stackrc

mariadb does not use DATABASE_USER = 'root' by default.
New versions of MySQL flavor MariaDB do not allow remote access for
'root' user by default.
This patch enables non-root users by loading DATABASE_USER variable
from stackrc (which is loaded by MYSQL_USER in localrc). It keeps 'root'
user as default for MySQL.

Change-Id: I15f5f7ca1fe91d97b3f5bf268982b133513e5e3c
(cherry picked from commit 6d3e6cfbfb)

inc/upgrade

@@ -33,8 +33,12 @@ function save_mysql_dbs {
         set +o xtrace &&
             source $dir/stackrc &&
             echo $MYSQL_PASSWORD)
+    local database_user=$(
+        set +o xtrace &&
+            source $dir/stackrc &&
+            echo ${DATABASE_USER:-root})
     for db in $DATABASES_TO_SAVE; do
-        mysqldump -uroot -p$mysql_pass $db >$SAVE_DIR/$db.sql.$release
+        mysqldump -u$database_user -p$mysql_pass $db >$SAVE_DIR/$db.sql.$release
     done
 }