nova: Set '[neutron] default_floating_pool' instead

In change I2ce8ff3d7c33a402b8af50182ec01f512859c388, we duplicated the
'default_floating_pool' option, found in the '[DEFAULT]' group, to the
'[neutron]' group. This allowed us to continue with our deprecation
plans for the former option, which should be retired along with
nova-network.

Update the nova lib module so it'll set the new option, we can safely
assume to be the correct one now that we've removed support for cells v1
and nova-network.

Change-Id: If9a02b640e6c2e1300c7b11b7552ba13c1496d79
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>
1 file changed