Ensure nullable=False for netid in packetfilters table

From the initial commit of NEC plugin, network_id of packetfilters
table is nullable=False, but in folsom_initial db migration script
nullable is set to True. This commit ensure nullable=False for
network_id in packetfilters table in any migration revision.

Change-Id: I2f1ebc16b57d8d6548255079c66d326d97fda5b6
Closes-Bug: #1229508
Merge "Ensure nullable=False for netid in packetfilters table"