Don't try validate TemplateResource properties in template_validate

If the user runs "heat template-validate ..." and one of the resources
is a TemplateResource we won't be able to validate the properties
of that resource as we have to instantiate the TemplateResource
to kick off downloading the template and conversion of parameters to
properties.
Note: validation will happen properly when the user creates the stack
      so this just delays the validation of one resource type by one step.

Change-Id: I6ce53a9e78ae470b54320862fe910eef9f603941
Closes-bug: #1236327
Merge "Don't try validate TemplateResource properties in template_validate" into milestone-proposed