Cleanup subnet creation in network api tests

This commit accomplishes 2 things, first it deduplicates the subnet
creation code by changing test_create_update_delete_network_subnet()
to use the common create_subnet() class method. Secondly the common
create_subnet() method is changed to properly use else after the for
loop instead of what was done before.

Change-Id: I04b0516e03eeb8b8f95d72259aa78f573db6ae45
Merge "Cleanup subnet creation in network api tests"