Make N1kv plugin net-delete more consistent w/ core

[Steven Hillman]

Currently the N1kV plugin has a check for subnets on network delete that reports an error when trying to delete the network if any subnets are present. This is a slight deviation from core plugin behavior which simply removes any associated subnets on network delete. Ideally we should enhance this behavior to be more consistent with core by having the plugin trigger the needed delete for any associated subnets on network delete.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/juno
Review: https://review.openstack.org/136873

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/138608

[OpenStack Infra (hudson-openstack)]

Change abandoned by Alan Pevec (<email address hidden>) on branch: stable/juno
Review: https://review.openstack.org/136873
Reason: Cherry pick from master when merged.
NB when doing that, make sure to keep the change-id
 https://wiki.openstack.org/wiki/StableBranch#Proposing_Fixes
 https://wiki.openstack.org/wiki/StableBranch#Change-Ids

[OpenStack Infra (hudson-openstack)]

Change abandoned by Saksham Varma (<email address hidden>) on branch: master
Review: https://review.openstack.org/138608
Reason: As Abhishek pointed out, monolithic plugin will be deprecated in Liberty, the ML2 plugin for N1Kv would take over.