doc: add some documentation on OpenStack datasource.

This just fills out some of the documentation on the OpenStack datasource.

doc/rtd/topics/datasources/openstack.rst

@@ -1,7 +1,41 @@
 OpenStack
 =========
 
-*TODO*
+This datasource supports reading data from the
+`OpenStack Metadata Service
+<http://docs.openstack.org/admin-guide/compute-networking-nova.html#metadata-service>`_.
+
+Configuration
+-------------
+The following configuration can be set for the datasource in system
+configuration (in `/etc/cloud/cloud.cfg` or `/etc/cloud/cloud.cfg.d/`).
+
+The settings that may be configured are:
+
+ * **metadata_urls**: This list of urls will be searched for an OpenStack
+   metadata service. The first entry that successfully returns a 200 response
+   for <url>/openstack will be selected. (default: ['http://169.254.169.254']).
+ * **max_wait**:  the maximum amount of clock time in seconds that should be
+   spent searching metadata_urls.  A value less than zero will result in only
+   one request being made, to the first in the list. (default: -1)
+ * **timeout**: the timeout value provided to urlopen for each individual http
+   request.  This is used both when selecting a metadata_url and when crawling
+   the metadata service. (default: 10)
+ * **retries**: The number of retries that should be done for an http request.
+   This value is used only after metadata_url is selected. (default: 5)
+
+An example configuration with the default values is provided as example below:
+
+.. sourcecode:: yaml
+
+  #cloud-config
+  datasource:
+   OpenStack:
+    metadata_urls: ["http://169.254.169.254"]
+    max_wait: -1
+    timeout: 10
+    retries: 5
+
 
 Vendor Data
 -----------