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
Merge "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." into R1.10