retain trailing newline from template files when using jinja2

sources.list was where this showed itself, but all rendered files
would have their newline stripped.

ChangeLog

@@ -4,6 +4,8 @@
  - expose uses_systemd as a distro function (fix rhel7)
  - fix broken 'output' config (LP: #1387340)
  - begin adding cloud config module docs to config modules (LP: #1383510)
+ - retain trailing eol from template files (sources.list) when
+   rendered with jinja (LP: #1355343)
 0.7.6:
  - open 0.7.6
  - Enable vendordata on CloudSigma datasource (LP: #1303986)

cloudinit/templater.py

@@ -89,9 +89,11 @@ def detect_template(text):
         return CTemplate(content, searchList=[params]).respond()
 
     def jinja_render(content, params):
+        # keep_trailing_newline is in jinja2 2.7+, not 2.6
+        add = "\n" if content.endswith("\n") else ""
         return JTemplate(content,
                          undefined=jinja2.StrictUndefined,
-                         trim_blocks=True).render(**params)
+                         trim_blocks=True).render(**params) + add
 
     if text.find("\n") != -1:
         ident, rest = text.split("\n", 1)