Use _is_dns_integration_supported for _delete_floatingip
Use _is_dns_integration_supported for _delete_floatingip

L3_NAT_dbonly_mixin._delete_floatingip still uses
utils.is_extension_supported to determine if DNS-specific methods should
be run rather than the self._is_dns_integration_supported method used
by _create and _update.

This causes issues with L3 service plugins like Cisco ACI where the core
plugin is ML2 but the L3 plugin does not implement the DNS methods.
When such a plugin sets _dns_integration to False, _create and _update
do not call the DNS-specific plugin methods, but _delete_floatingip does
 which leads to an exception being raised.

This patch changes _delete_floatingip to use the same method for
determining DNS support as _create_floatingip and _update_floatingip.

Change-Id: Ieb2db6cb96c5d6aae166aa7f0d2d481b2a3690f9
Closes-Bug: #1702693

openstack/neutron,openstack/neutron,eayunstack/neutron,noironetworks/neutron,huntxu/neutron,mahak/neutron,mahak/neutron,noironetworks/neutron,openstack/neutron,mahak/neutron,eayunstack/neutron,huntxu/neutron