Fix retrieval of shared networks

Bug 1130171

This patch adds a new tag_scope, 'shared', in order to allow
the NVP plugin to retrieve also shared networks from NVP when
listing networks. This way it won't appear as Quantum has a
network without a corresponding lswitch in NVP

Change-Id: I24f950d54c0b9e582c92e8008e75ebd00f6c9fe3
Merge "Fix retrieval of shared networks"