Introduce announcer initialization module api

[olix0r]

In the same way that we load protocol and namer support at runtime, we should introduce a pluggable Announcer api. The Linkerd main should load a list of io.bouyant.linkerd.AnnouncerInitializer modules. Something to the effect of:

trait AnnouncerInitializer {
  def kind: String
  def announce(server: ListeningServer): Closable
}

Then, the configuration format should be extended to admit a list of announcers on servers. Supporting a list of announcers is critical to "multiple registration" schemes, which become very important during migrations (i.e. between zk clusters, or from one sd backend to another, etc).

Furthermore, in this announcing regime, we are not supporting announcing of a service's port, but of the router's server (which will forward to the server's port). This primarily benefits linker-to-linker configurations like:

For example

routers:
- protocol: http
  label: upstream
  baseDtab: /http => /$/inet/127.1/8881
  servers:
  - port: 8080
    ip: 0.0.0.0
    - announcers:
      - kind: io.l5d.serverset
        path: /services/userservice
        zkAddrs:
        - host: zkcluster.buoyant.io
      - kind: io.l5d.serverset
        path: /services/userservice
        zkAddrs:
        - host: otherzkcluster.buoyant.io

- protocol: http
  label: downstream
  baseDtab: ...
  servers:
  - port: 4140

[olix0r]

This is done! #368