Fix secgroups exercise on postgres

This should fix the failing (but non-voting) postgres gate tests.

Why does postgresql change the value '0.0.0.0/00' to '0.0.0.0/0'?
Clearly the correct value for the network CIDR bits is with only
one zero but even an incorrect value shouldn't be changing.

SQLalchemy is given this for the column:
        Column('cidr', String(length=255)),

Change-Id: Ib19dad23789654664e90518087e5a462fa8b8034