Don't validate parameters when loading a stack

All kinds of Frankentemplates can now be stored in the database. We don't
want to refuse to load them again. Only validate the parameters when we
actually validate the stack.

Change-Id: I8bff116f1a9c16be2bd10a8da685ef9571e1101f
Closes-bug: #1354962
Merge "Don't validate parameters when loading a stack"