Restore NOT NULL constraint lost by earlier migrations

Bring back nullable=False for columns in tables poolstatisticss
and ipsec_site_connections that disapeared because of
migrations abc88c33f74f and 338d7508968c correspondingly.

Closes-bug: #1288681

Change-Id: Idc4608616c2b4c06e7404157b2f708d46e91accc
Merge "Restore NOT NULL constraint lost by earlier migrations"