Maint: neutronclient exceptions from a more appropriate module

When nova wants to use neutronclient exceptions, it should import
neutronclient.common.exceptions, but there are some cases they import
nova.network.neutronv2, which indeed can import neutronclient exceptions,
but may confuse people. This patch fixes them.

Change-Id: Ief388fc631d18c709a629980143d53f240a2ed2b
Merge "Maint: neutronclient exceptions from a more appropriate module"