creating a default subnet

This changes how SwitchManager handles subnets so that when there are
no configured subents, it returns a default subnet that spans all ports
in the network and spans all IP addresses, i.e., 0.0.0.0/0.

This makes it so that HostTracker will discover all hosts by default
instead of only discovering hosts in configured subents. This, in turn,
makes SimpleForwarding forward traffic correctly without any
configuration.

Change-Id: Ice31b6035b970dd69fc69df3d9560b4aac41630d
Signed-off-by: Colin Dixon <ckd@us.ibm.com>