The --use-prefix-delegation argument when creating a
subnet should not take an argument of True/False, it
should simply trigger the code to set subnetpool_id
value to 'prefix_delegation'. Change action to correct
this.
Added unit test to cover missing checks.
Related-bug: #2028159
Change-Id: Ib7ee80100327b8611d4a354c7f4eb0e696c953da