IPv4 DAD is non-existent in Linux or its failure is silent, so we
never needed to catch and ignore it. On the other hand IPv6 DAD
failure is explicit, hence comes this change.
This of course leaves the metadata service dead on hosts where
duplicate address detection failed. But if we catch the
DADFailed exception and delete the address, at least other
functions of the dhcp-agent should not be affected.
With this the IPv6 isolated metadata service is not redundant, which
is the best we can do without a redesign.
Also document the promised service level of isolated metadata.
Added additional tests for the metadata driver as well.
Change-Id: I6b544c5528cb22e5e8846fc47dfb8b05f70f975c
Partial-Bug: #1953165
2aee961ab6