HOT template to deploy servers to existing network

Add a HOT template to deploy two servers into an existing neutron network and
assign floating IPs to them. The template uses just native resources.
There is no special assumption on the image used for deployment since no
configuration inside the instance is done. The template is really just for
showing the base neutron configuration.

Change-Id: I214f57e83b134fd6a1c36d121f43669bf609677a