New container sync configuration option

Summary of the new configuration option:

The cluster operators add the container_sync middleware to their
proxy pipeline and create a container-sync-realms.conf for their
cluster and copy this out to all their proxy and container servers.
This file specifies the available container sync "realms".

A container sync realm is a group of clusters with a shared key that
have agreed to provide container syncing to one another.

The end user can then set the X-Container-Sync-To value on a
container to //realm/cluster/account/container instead of the
previously required URL.

The allowed hosts list is not used with this configuration and
instead every container sync request sent is signed using the realm
key and user key.

This offers better security as source hosts can be faked much more
easily than faking per request signatures. Replaying signed requests,
assuming it could easily be done, shouldn't be an issue as the
X-Timestamp is part of the signature and so would just short-circuit
as already current or as superceded.

This also makes configuration easier for the end user, especially
with difficult networking situations where a different host might
need to be used for the container sync daemon since it's connecting
from within a cluster. With this new configuration option, the end
user just specifies the realm and cluster names and that is resolved
to the proper endpoint configured by the operator. If the operator
changes their configuration (key or endpoint), the end user does not
need to change theirs.

DocImpact

Change-Id: Ie1704990b66d0434e4991e26ed1da8b08cb05a37
Merge "New container sync configuration option"