If the API client updates a network object with subnets and does not send a subnet_uuid, we were allocating a new UUID. If the subnet already exists, we should reuse the subnet_uuid. Now, we read from cassandra before generating a new UUID.

Change-Id: I13426663c8a2a8e79fbf170dffb81c726506cee1
Closes-Bug: 1381779
(cherry picked from commit c2f71af5f2e9a6f31ce65e48c0e4a4c62e82f0c8)
(cherry picked from commit 0f67b7da211fb961ffdf1f3b671d2fd7d3b49124)