Support utf8 in external file alertmanager config

This could have been achieved either by switching to stringData, or doing
`std.base64(std.encodeUTF8($._config.alertmanager.config))` as per
google/jsonnet#575

I went with the former, because it's:
1. Easier to read existing config
2. Consistent with the way jsonnet object-based config is written just above

jsonnet/kube-prometheus/alertmanager/alertmanager.libsonnet

@@ -81,7 +81,8 @@ local k = import 'ksonnet/ksonnet.beta.4/k.libsonnet';
         .withStringData({ 'alertmanager.yaml': std.manifestYamlDoc($._config.alertmanager.config) }) +
         secret.mixin.metadata.withNamespace($._config.namespace)
       else
-        secret.new('alertmanager-' + $._config.alertmanager.name, { 'alertmanager.yaml': std.base64($._config.alertmanager.config) }) +
+        secret.new('alertmanager-' + $._config.alertmanager.name, {})
+        .withStringData({ 'alertmanager.yaml': $._config.alertmanager.config }) +
         secret.mixin.metadata.withNamespace($._config.namespace),
 
     serviceAccount: