juju-update-bootstrap-address plugin

[Impact]
Users of juju environments with failed bootstrap nodes are unable to restore the environment (backup/recovery)

[Test Case]
In a failed environment which has had a new bootstrap node provisioned and the original state restored from backup as described in bug #1254729

juju update-bootstrap

Existing machines will be updated to report into the new juju bootstrap node.

[Regression Potential]
Minimal; this new plugin is separate to the existing codebase.

[Original Bug Report]
The purpose of this plugin is to update a bootstrap address on all other running juju machines given that the bootstrap node has failed completely, and another bootstrap node has been brought up using the plugins described in bug 1254574 and bug 1254577.

Fundamentally this should be reasonably simple:

run 'juju status' and parse the results to get a list of machines to ssh into (excluding machine 0).

for each machine:
  ssh to that machine
    stop the machine agent and each unit agent
    edit the udp address in the /etc/rsyslog configuration for each juju agent
    restart rsyslog
    edit the api addresses and state addresses stored for the machine and each unit agent
    start the machine agent and each unit agent