Create fallback for missing `deployment` on domain

Came across this when debugging something else.  It appears some domain
docs don't have a `deployment` field - snapshots, I think.  This should
prevent pillow errors like this:
https://www.commcarehq.org/hq/pillow_errors/edit_errors/?error=53475
Merge pull request #5160 from dimagi/domain-pillow

Create fallback for missing `deployment` on domain