Fixed bug with regards to subnet lookups.

Fixed bug with regards to subnet lookups.
Neutron returns all subnets regardless of visibility which cause problems
within routers if there is another subnet with the same name attached to
a different network.

JIRA: SNAPS-304

In addition, this patch contains two other minor fixes.
  launch_utils.py - raise an exception when the creator is not properly instantiated
  network.py - allow fixed IPs to be none.

Change-Id: Ib343074d925be4592a713727a03d5b531890eada
Signed-off-by: spisarski <s.pisarski@cablelabs.com>