This commit now allows user to specify 'prefix_delegation'
as a Subnetpool during Subnet creation by using the new
--use-prefix-delegation option so that the IPv6
prefixes can be delegated to routers set up by the
cloud admins.
Change-Id: I67e5d81c4155db2e3c5c41ee1df77f2d77a17689
Closes-Bug: #1513894