Fix rendering of obscure MAAS file drivers.yaml

The existing drivers.yaml rendered by the MAAS chart is missing the
top-level 'drivers' key, so it doesn't actually work. This change fixes
the rendering of the file, and adds a comment in values.yaml about where
to look for additional information about where and how the file is used:

https://github.com/maas/maas/blob/2.3.5/src/maasserver/third_party_drivers.py

Change-Id: I940c8a57d3e404a101de5c1ea92f8a467319dbaa

charts/maas/templates/etc/_drivers.yaml.tpl

@@ -14,10 +14,16 @@
 # limitations under the License.
 */}}
 {{- if .Values.conf.drivers }}
-{{- range .Values.conf.drivers -}}
+drivers:
+{{- range .Values.conf.drivers }}
 - comment: {{ .comment }}
+{{- if .key_binary }}
   key_binary: !!binary |
 {{ .key_binary | indent 4 }}
+{{- end }}
+{{- if .repository }}
+  repository: {{ .repository }}
+{{- end }}
   modaliases:
 {{ .modaliases | toYaml | indent 4 }}
   module: {{ .module }}

charts/maas/values.yaml

@@ -257,6 +257,8 @@ conf:
   drivers: null
   #### If you populates drivers, it will replace the 3rd party driver
   #### info that comes with MaaS. see structure below if it is needed
+  #### Additional context about the use of this file can be found here:
+  #### https://github.com/maas/maas/blob/2.3.5/src/maasserver/third_party_drivers.py
   # - blacklist: string    # a kernel module to blacklist from loading if needed
   #   comment: string       # free form comment
   #   key_binary: |    # The GPG key for the repo holding the package, base64 encoded, non-ascii armored