Schedule net to a DHCP agt on subnet create

Schedule net to a DHCP agt on subnet create

Change the DHCP notifier behavior to schedule a network
to a DHCP agent when a subnet is created rather than
waiting for the first port to be created.

This will reduce the possibility to get a VM port created
and have it send a DHCP request before the DHCP agent is
ready. Before, the network would be scheduled to an agent
as a result of the API call to create the VM port, so the
DHCP port wouldn't be created until after the VM port.
After this patch, the network will have been scheduled to
a DHCP agent before the first VM port is created.

There is still a possibility that the DHCP agent could be
responding so slowly that it doesn't create its port and
activate the dnsmasq instance before the VM sends traffic.
A proper fix will ensure that the dnsmasq instance is
truly ready to serve requests for a new port will require
significantly more code for barriers (either on the subnet
creation, port creation, or the nova boot process) are too
complex to add this late in the cycle.

This patch also eliminates the logic in the n1kv plugin that
was already doing the same thing.

Closes-Bug: #1431105
Change-Id: I1c1caed0fdda6b801375a07f9252a9127058a07e