Chef Failing on Prometheus Alert Managers

Chef is failing to run on `alerts-01-inf-gprd.c.gitlab-production.internal` and `alerts-02-inf-gprd.c.gitlab-production.internal` with the following error:

```ruby
  * template[/opt/prometheus/alertmanager/alertmanager.yml] action create[2018-09-18T23:32:58+00:00] INFO: Processing template[/opt/prometheus/alertmanager/alertmanager.yml] action create (gitlab-alertmanager::default line 70)


    ================================================================================
    Error executing action `create` on resource 'template[/opt/prometheus/alertmanager/alertmanager.yml]'
    ================================================================================

    Chef::Mixin::Template::TemplateError
    ------------------------------------
    undefined method `[]' for nil:NilClass

    Resource Declaration:
    ---------------------
    # In /var/chef/cache/cookbooks/gitlab-alertmanager/recipes/default.rb

     70: template node["alertmanager"]["flags"]["config.file"] do
     71:   source "alertmanager.yml.erb"
     72:   owner node["prometheus"]["user"]
     73:   group node["prometheus"]["group"]
     74:   mode "0644"
     75:   variables(conf: alertmanager_conf)
     76:   notifies :hup, "runit_service[alertmanager]"
     77: end
     78:

    Compiled Resource:
    ------------------
    # Declared in /var/chef/cache/cookbooks/gitlab-alertmanager/recipes/default.rb:70:in `from_file'

    template("/opt/prometheus/alertmanager/alertmanager.yml") do
      action [:create]
      retries 0
      retry_delay 2
      default_guard_interpreter :default
      source "alertmanager.yml.erb"
      variables {:conf=>{"slack"=>{"channel"=>"#alerts", "api_url"=>"https://hooks.slack.com/services/XXXXX/XXXXX/XXXXX"}, "pagerduty"=>{"service_key"=>"XXXXX", "low_prio_service_key"=>"XXXXX"}}}
      declared_type :template
      cookbook_name "gitlab-alertmanager"
      recipe_name "default"
      owner "prometheus"
      group "prometheus"
      mode "0644"
      path "/opt/prometheus/alertmanager/alertmanager.yml"
      verifications []
    end

    Template Context:
    -----------------
    on line #225
    223: - name: dead_mans_snitch
    224:   webhook_configs:
    225:   - url: "https://nosnch.in/<%= @conf['snitch']['api_key'] %>"
    226:     send_resolved: false

    Platform:
    ---------
    x86_64-linux

[2018-09-18T23:32:58+00:00] INFO: Running queued delayed notifications before re-raising exception

Running handlers:
[2018-09-18T23:32:58+00:00] ERROR: Running exception handlers
  - PrometheusHandler
Running handlers complete
[2018-09-18T23:32:58+00:00] ERROR: Exception handlers complete
Chef Client failed. 14 resources updated in 31 seconds
[2018-09-18T23:32:58+00:00] FATAL: Stacktrace dumped to /var/chef/cache/chef-stacktrace.out
[2018-09-18T23:32:58+00:00] FATAL: Please provide the contents of the stacktrace.out file if you file a bug report
[2018-09-18T23:32:58+00:00] ERROR:

Chef::Mixin::Template::TemplateError (undefined method `[]' for nil:NilClass) on line #225:

223: - name: dead_mans_snitch
224:   webhook_configs:
225:   - url: "https://nosnch.in/<%= @conf['snitch']['api_key'] %>"
226:     send_resolved: false
```

Story points: 2.0