Merge ~arnd-arndnet/cloud-init:document_gce_datasource into cloud-init:master

[Arnd]

doc: document GCE datasource.

Add some minimal documentation for GCE datasource.

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Unmerged commits

c21ff9a... by Arnd Hannemann <email address hidden> on 2017-09-21

doc: include gce datasource in toc

4c1971b... by Arnd Hannemann <email address hidden> on 2017-08-21

doc: document gce datasource

Add some minimal documentation for gce datasource.

diff --git a/doc/rtd/topics/datasources.rst b/doc/rtd/topics/datasources.rst
index a60f5eb..7e2854d 100644
--- a/doc/rtd/topics/datasources.rst
+++ b/doc/rtd/topics/datasources.rst
@@ -94,5 +94,6 @@ Follow for more information.
    datasources/ovf.rst
    datasources/smartos.rst
    datasources/fallback.rst
+   datasources/gce.rst
 
 .. vi: textwidth=78
diff --git a/doc/rtd/topics/datasources/gce.rst b/doc/rtd/topics/datasources/gce.rst
new file mode 100644
index 0000000..3b1b50b
--- /dev/null
+++ b/doc/rtd/topics/datasources/gce.rst
@@ -0,0 +1,22 @@
+.. _datasource_gce:
+
+Google Compute Engine
+=====================
+
+The GCE datasource gets its data from the internal compute metadata server.
+Metadata can be queried at the URL
+``http://metadata.google.internal/computeMetadata/v1/``
+from within an instance.
+
+(for reference see
+https://cloud.google.com/compute/docs/storing-retrieving-metadata#querying)
+
+Currently the default project and instance level metadatakeys keys
+``project/attributes/sshKeys`` and ``instance/attributes/ssh-keys`` are merged
+to provide ``public-keys``.
+
+``user-data`` and ``user-data-encoding`` can be provided to cloud-init by
+setting those custom metadata keys for an *instance*.
+
+
+.. vi: textwidth=78