mariadb: make security_reset.expect's timeout configurable

Under heavy load, when the security_reset script is being run to set
up root password, it might happen that the mysql server doesn't get a
change to evaluate the SQL statement sent by security_reset.expect
before the hardcoded 10 seconds timeout is reached.

Allow the timeout to be overriden via env variable DB_MAX_TIMEOUT,
which is already used as a timeout variable in extend_start.sh

Change-Id: If82c6daca82a48d899605829dc99cf4a352cddcd
Closes-Bug: #1775864

docker/mariadb/security_reset.expect

@@ -1,6 +1,6 @@
 #!/usr/bin/expect -f
 
-set timeout 10
+if [catch {set timeout $env(DB_MAX_TIMEOUT)}] {set timeout 10}
 spawn mysql_secure_installation
 expect {
   timeout { send_user "\nFailed to get 'Enter current password for root (enter for none):' prompt\n"; exit 1 }