Bug 2214 - Unable to set the external_ids field of the Qos object

Fix typo on external_ids for QoS table, to conform with
http://openvswitch.org/ovs-vswitchd.conf.db.5.pdf page 38.

Change-Id: I97ac25bf1355b18d9944de3e78bbe6edadd3399d
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>