SNAPS Stack creators can now return SNAPS network creators.

SNAPS Stack creators can now return SNAPS network creators.

As Heat Stacks are responsible for spawning objects in OpenStack,
the class OpenStackHeatStack which is responsible for applying and
managing the state of a stack now can retrieve OpenStackNetwork
objects for the networks created in the stack for clients who would
like to query the networks and subnets or update them outside of
Heat.

JIRA: SNAPS-171

Change-Id: I9bf0b81d4f7bfeb1b6392f345022c7d9a57d0415
Signed-off-by: spisarski <s.pisarski@cablelabs.com>