null: machine addresses may override bootstrap hostname

Recent work to have machine agents publish their machine addresses has caused a problem for the manual provider. The bootstrap node currently does not record the host/IP bootstrapped with as being public, and so any machine address that comes along may override it as being the "most public address".

For example, if I bootstrap on one network interface (10.0.3.1), and wait a while, the machine agent will eventually record all its addresses; they're all non-public, so any one of them may win (and in my case, if you look at the bottom, 192.168.122.1 was selected).

andrew@blank:~/code/src/launchpad.net/juju-core/cmd$ juju bootstrap --upload-tools
Logging to /var/log/cloud-init-output.log on remote host
Running apt-get update
Installing package: git
Installing package: cpu-checker
Installing package: mongodb-server
Starting MongoDB server (juju-db)
Bootstrapping Juju machine agent
Starting Juju machine agent (jujud-machine-0)

andrew@blank:~/code/src/launchpad.net/juju-core/cmd$ juju status
environment: "null"
machines:
  "0":
    agent-state: pending
    dns-name: 10.0.3.1
    instance-id: 'manual:'
    series: saucy
    hardware: arch=amd64 cpu-cores=4 mem=15952M
services: {}

andrew@blank:~/code/src/launchpad.net/juju-core/cmd$ juju status
environment: "null"
machines:
  "0":
    agent-state: down
    agent-state-info: (started)
    agent-version: 1.17.2.1
    dns-name: 192.168.122.1
    instance-id: 'manual:'
    series: saucy
    hardware: arch=amd64 cpu-cores=4 mem=15952M
services: {}