Fix log and test for NeutronAPI.update_port_dhcp_opts

The unit tets for NeutronAPI.update_port_dhcp_opts were raising the
wrong exception, and would pass regardless of the behaviour of
update_port_dhcp_opts.

This patch causes the mock to raise a neutron_client exception and
correctly asserts that ironic.common.neutron.NeutronAPI converts the
neutron client expcetion to an ironic exception.

It also adjusts the LOG behavior of this method to properly record
the exception traceback, which could be useful in determining the
failure in the neutron client.

Change-Id: I6f231b6a823386709b5778d140d6aa48ae4997ce
Merge "Fix log and test for NeutronAPI.update_port_dhcp_opts"