add reading of 'seedfrom' via DataSourceNoCloud's DataSourceConfig

What this does is provide an second DataSource that could use the
kernel command line url=.  For example:
  ro root=/dev/vda url=http://example.com/i-abcdefg/

http://example.com/i-abcdefg/ would contain:
datasource:
  NoCloud:
    # default seedfrom is None
    # if found, then it should contain a url with:
    #    <url>/user-data and <url>/meta-data
    # seedfrom: http://my.example.com/i-abcde
    seedfrom: http://example.com/i-abcdefg/

Then, the NoCloudNet DataSource would find that seedfrom config
and consume data at
  http://example.com/i-abcdefg/user-data
and
  http://example.com/i-abcdefg/meta-data

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/examples/cloud-config-datasources.txt b/doc/examples/cloud-config-datasources.txt
index 63a6cfc4..d10dde05 100644
--- a/doc/examples/cloud-config-datasources.txt
+++ b/doc/examples/cloud-config-datasources.txt
@@ -24,3 +24,10 @@ datasource:
     consumer_key: Xh234sdkljf
     token_key: kjfhgb3n
     token_secret: 24uysdfx1w4
+
+  NoCloud:
+    # default seedfrom is None
+    # if found, then it should contain a url with:
+    #    <url>/user-data and <url>/meta-data
+    # seedfrom: http://my.example.com/i-abcde
+    seedfrom: None