Implement Midonet Juno Network Api calls

Implement Midonet Juno Network Api calls

This patch is the first in a series of converting the current midonet plugin
code to the new model described in blueprint midonet-plugin-juno. This patch
focuses specifically on the "network" handlers.

This patch also updates the midonet UT to handle the new objects that need
to be mocked with the new model.

Change-Id: I056d698ce2e312527e756ff56c962cff83a6bc04
Partially-Implements: blueprint midonet-plugin-juno