Networks are not scheduled to DHCP agents for Cisco N1KV plugin

With the config option 'network_auto_schedule = False' in
neutron.conf, networks do not get scheduled to available DHCP
agents. The fix is to explicitly schedule the network as part
of the subnet creation flow.

Change-Id: Id1f94a5844111a916f984d6dd7bda2cb4e11e1ee
Closes-Bug: #1356609
Merge "Networks are not scheduled to DHCP agents for Cisco N1KV plugin"