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

cloudinit/DataSourceNoCloud.py

@@ -67,6 +67,13 @@ class DataSourceNoCloud(DataSource.DataSource):
             found.append(self.seeddir)
             log.debug("using seeded cache data in %s" % self.seeddir)
 
+        # if the datasource config had a 'seedfrom' entry, then that takes
+        # precedence over a 'seedfrom' that was found in a filesystem
+        # but not over external medi
+        if 'seedfrom' in self.ds_cfg and self.ds_cfg['seedfrom']:
+            found.append(["ds_config"])
+            md["seedfrom"] = self.ds_cfg['seedfrom']
+
         fslist = util.find_devs_with("TYPE=vfat")
         fslist.extend(util.find_devs_with("TYPE=iso9660"))
 

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