subnet_uuid may not be present in the subnet dict. Use get() to access it.

Change-Id: Ided70dc28a225566d85b7ca4642bb4fc20f0940c
Closes-Bug: 1372307
(cherry picked from commit c93beb957cafc886c3d6ef6a746242185e32e93b)
Merge "subnet_uuid may not be present in the subnet dict. Use get() to access it." into R1.10