Fix merge issues on stdout branch

The act of committing a PR that including part of these changes and then
reverting it meant that some of the changes from the original branch
didn't land in this new PR.   This commit adds the missing bits.

Lesson learned:  Create a new branch when submitting a PR on a
previously reverted PR

awx/api/renderers.py

@@ -45,6 +45,9 @@ class PlainTextRenderer(renderers.BaseRenderer):
             data = unicode(data)
         return data.encode(self.charset)
 
+class DownloadTextRenderer(PlainTextRenderer):
+    format = "txt_download"
+
 class AnsiTextRenderer(PlainTextRenderer):
 
     media_type = 'text/plain'