Fix autocreate subnet ownership problem

DHC-1560

When a user create a network in a tenant he is not member of
passing the --tenant_id option to the net-create command, the
network is created with the right ownership, but the subnet
we are creating automatically is owned by the tenant from the
command credentials rather than the value passed explicitly
as the owner.

Change-Id: I48810b87c6603f8d511a7fd4b4e64c1f4370a56e
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>