Upgrade sensu and module

Successfully provisioned a monitoring-1 and backup-box-1 with this,
after provisioning with master

Settings required for latest puppet module

 - Be explicit about the api
 - Need to specify a version of the sensu plugin gem
 - The puppet module changed the default vhost to not include a slash -
   https://github.com/sensu/sensu-puppet/pull/184

This unfortunately broke our setup, which expected a slash in the vhost.
Add uchiwa puppet module and config

Uchiwa replaces the sensu dashboard

Uchiwa defaults to using port 3000, so set the nginx alerts upstream to use that
Merge pull request #671 from alphagov/sensu-upgrade

Upgrade sensu