Remove validation of nested stack for preview

At preview time, resources have not been created and certain validation
do not apply (such as validating properties acquired from other
resources attributes), so this patch removes the validation of nested
stacks.

Closes-Bug: 1392455
Change-Id: I39be064c775369f1180487f4b48184e288734c49
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
Merge "Mark stack_domain_admin_password as a secret option"
Merge "Remove validation of nested stack for preview"