Adds network model and network info cache.

The next merge will prepopulate the cache, and use the model to keep the
cache up to date.
I realize "cache" is a bit of a stretch for what this is doing.

blueprint network-info-model
blueprint compute-network-info

Change-Id: I0f0f4ba3de1310e1ff89239dab6ea8e24c85f2c8

nova/exception.py

@@ -317,6 +317,10 @@ class InvalidCPUInfo(Invalid):
     message = _("Unacceptable CPU info") + ": %(reason)s"
 
 
+class InvalidIpAddressError(Invalid):
+    message = _("%(address)s is not a valid IP v4/6 address.")
+
+
 class InvalidVLANTag(Invalid):
     message = _("VLAN tag is not appropriate for the port group "
                 "%(bridge)s. Expected VLAN tag is %(tag)s, "