Mark stack_domain_admin_password as a secret option

Add secret=True to option stack_domain_admin_password,
so that will avoid logging this option in engine.log.

Closes-Bug: #1392162
Change-Id: I04071f37a0053b969dc31a69e62ba9abb075c590
(cherry picked from commit 0307d0267713ed18309ecea5359f002d388eaf34)
Merge "Mark stack_domain_admin_password as a secret option" into stable/juno