Add _security_group_ensure_default() DBAPI method

According to the convention we use for DB API methods, every public
method should have a private counterpart. The difference between
those two is that the latter accepts an optional 'session' argument,
which makes it possible for the method to participate in the existing
database transaction.

security_group_ensure_default() was called inside instance_create()
and thus implicitly created a new concurrent db transaction, which
is now fixed by making a call to a private counterpart and passing
the session instance.

Closes-Bug: #1362233

Change-Id: Ifa1b6430328fde1ab0984f726bd8392bbd3f7ee5
Merge "Add _security_group_ensure_default() DBAPI method"