network: Fix subnet_allocation extension check.
network: Fix subnet_allocation extension check.

This code checked for a "subnetpools" extension.  While it's commonly
referred to that way, and may have originally been called that, the
API extension is actually "subnet_allocation".  Nobody has probably
noticed this error as the default neutron jobs run with the
api_extensions configuration set to "all", so all checks like this
would return True.

Change-Id: I646b5e988bc6a9053d5361d81ed4c166fc670c7a
Signed-off-by: Russell Bryant <b61ba72562ef1f6718e4f653428a87ed07b576b8@redhat.com>

sebrandon1/tempest,cisco-openstack/tempest,Juniper/tempest,vedujoshi/tempest,LIS/lis-tempest,cisco-openstack/tempest,zsoltdudas/lis-tempest,Juniper/tempest,zsoltdudas/lis-tempest,vedujoshi/tempest,openstack/tempest,sebrandon1/tempest,LIS/lis-tempest,masayukig/tempest,bigswitch/tempest,Tesora/tesora-tempest,Tesora/tesora-tempest,masayukig/tempest,openstack/tempest,bigswitch/tempest