Make allocation_pools attribute of subnet updateable by PUT

The review[1] makes the allocation_pools attribute of subnet updateable by
PUT, we should keep changes synchronized with the documentation.
[1]: https://review.openstack.org/#/c/62042/
Updated the common.ent file to remove the non-updateable attribs

Change-Id: I3b5dd887124494b74f00d8105be23fe8dcbdc659
Closes-Bug: #1328340