Do no re-validate parameters for existing stacks

This patch disables validation of stack parameters values when
loading existing stacks from the database. The old behavior was
to always validate stack parameter values when initializing a
Stack object. However, this causes problems especially with
custom constraints when a constraint could be fulfilled during
stack creation but cannot be fulfilled at a later point (e.g.
when a flavor or keypair has been deleted). In such a case,
the existing stack became completely unusable.

This patch changes the behavior to not do parameter validation
for existing stacks. The assumption is that validation at stack
creation time should be sufficient to make sure the stack is valid,
so only valid stacks end up in the database. Therefore, validating
parameters again when loading stacks is not really required.

Closes-Bug: #1314401

Cherry-pick from review https://review.openstack.org/#/c/94329
(cherry picked from commit 349a6f04b53309f7fa8f3225f98d41670888d20c)

Conflicts:

        heat/tests/test_parser.py

Change-Id: I0947c2dcfb9db4c81b07de582aab86f262d0c008
Merge "Do no re-validate parameters for existing stacks" into stable/icehouse