Allow init command to set options from environment variables

The manifest URL and mirror location can be specified in environment
variables which will be used if the options are not passed on the
command line

Change-Id: Ida87968b4a91189822c3738f835e2631e10b847e

subcmds/init.py

@@ -118,6 +118,10 @@ to update the working directory files.
                  dest='config_name', action="store_true", default=False,
                  help='Always prompt for name/e-mail')
 
+  def _RegisteredEnvironmentOptions(self):
+    return {'REPO_MANIFEST_URL': 'manifest_url',
+            'REPO_MIRROR_LOCATION': 'reference'}
+
   def _SyncManifest(self, opt):
     m = self.manifest.manifestProject
     is_new = not m.Exists