Correct error msg when resource or output are empty

When resource or output body is empty, we get traceback
instead of error message. This is wrong, so this patch
handle such situation by raising StackValidationFailed
exception. Furthermore, if output body is not a mapping,
we got traceback too.

Change-Id: I27f3a732480a851fd2eceb83e5fcc850ee59bb73
Closes-bug: #1386242
Add functional test for update of provider resource

Adds a test where we update a nested stack definition, to ensure
the implementation signature is correctly recognised as changed
so the update is applied in-place to the nested stack.

Change-Id: I2fbbf749d7265d26fd7104036fe61d3e855ca7b7
Related-Bug: #1389499
Merge "Correct error msg when resource or output are empty"
Merge "Add functional test for update of provider resource"