core: Allow anonymous QoS in DB

Relaxed the NOT NULL constraints on the name and storage_pool_id
columns in the network_qos table, to allow for anonymous QoS entities
to be persisted.

Change-Id: Idc1041fcd790ff1f32b87bf68e8948cb16bcf623
Signed-off-by: Lior Vernia <lvernia@redhat.com>