Change hard coded numbers to constants in security group tests

Security groups tests in Neutron contain a lot of hard coded HTTP return codes
and protocol numbers. These should be changed to use constants.

Change-Id: Ibecff3821c54f12848a05648f35381a0c73a0896
Fixes: Bug 1218928
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 "Change hard coded numbers to constants in security group tests"
Merge "Ensure nullable=False for netid in packetfilters table"