Fix SecurityGroupExists error when booting instances

Make sure we properly handle the case when multiple threads check
and create the default security group, if it hasn't existed yet.
One thread will succeed and others will fail (we have a unique
constraint in db to prevent a race condition), so just handle the
exception to let instance boot proceed.

Steps to reproduce:

1) install OpenStack with Rally using DevStack:

    https://github.com/stackforge/rally/tree/master/contrib/devstack

2) fetch the Rally task:

    http://boris-42.github.io/boot.yaml

3) run it

    rally -v task start boot.yaml

Closes-Bug: #1370782

Change-Id: I414637a31b0b540fefa6c1314be6e973e2d4a178
Merge "Fix SecurityGroupExists error when booting instances"