Make swift FnGetAtt fault tolerant and block less

head_container is now only called for required attributes
and will tolerate failure if there is an issue with the underlying
container.

This was preventing deleting of stacks with swift resources.

Possibly this is eligible for backporting to milestone-proposed.

Change-Id: I2f489126957f11924943955a7f63a2ebc45f1911
Fixes: bug #1160584
Merge "Make swift FnGetAtt fault tolerant and block less"