cache-server scaffolding

[p0lyn0mial]

Summary

This PR creates an empty server/binary that follows options/config/server pattern.
The server can be run in two modes, as a standalone binary or can be embedded into the existing server.
This is the first part, in the following PRs I'm planning to wire the generic api server, the etcd, and the apiextention server.

The cache server hosts data/resources that are required by shards.
It serves as a cache helping to reduce the storage that would have to
be copied onto every shard otherwise.

The actual group of shards that will use this server should be part of 
the topology. For example, it can be used only by shards that are in 
the same geographical region.

On a high level, the server exposes two HTTP paths. The first one is 
used by the shards for getting all resources. The second one is used 
by individual shards to push data they wish to be shared.

There are no limits on the types of data this server hosts. The rule of 
thumb is that they must be common for a larger group of shards. 
For example the root APIs. 

Related issue(s)

part of 4. implement replication controller / cache server in #1225

[p0lyn0mial]

/assign @stevekuznetsov @sttts

[openshift-ci]

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[stevekuznetsov]

#342