get_security_groups now creates default security group

If one called get_security_groups() previously the default security
group would not be created until the tenant created a network. This
exposed a bug when an admin creates a shared network and a tenant tried
to boot a vm on it without having any networks.

Fixes bug 1171997

Change-Id: I899203424c03353a1ee6567c9ddbaecd7dc25001
Merge "get_security_groups now creates default security group"