Ensure to remove auto delete ports from OFC in NEC plugin

When deleting a network, dhcp ports are removed automatically
in db_plugin. This causes a failure network deletion since the
corresponding port exists on OFC and the network on OFC cannot
be deleted unless a subnet on the network is deleted explicitly.
This commit gets auto-delete ports and delete the corresponding
ports on the controller before deleting the network.

Fixes bug #1190163

TestNecPluginOfcManager class is borrowed from commit 81ce9890ce
in master branch.

Change-Id: Ifffaa1a28b5fac0b0a98036a926550255cc25c9d
(cherry picked from commit da6816c6c703be68f5a0ce537364bc096ec2bd9d)
Merge "Ensure to remove auto delete ports from OFC in NEC plugin" into stable/grizzly