Correct cloner shim missing project error

When asked to clone a project which is not present in REPO_SRC_DIR,
the error message should suggest adding the missing project's name
to the required-projects list for the job rather than its file path.

Also, while we're at it, implement Clint's suggestion from
Ia94d03a9fe59677129e23e849e0437200a66f4c3 to embed REPO_SRC_DIR's
value into the error rather than hard-coding an OpenStack-specific
implementation value.

Change-Id: I068ab7df86338794b71b23eab748e01ccd0d90f3

roles/fetch-zuul-cloner/templates/zuul-cloner-shim.py.j2

@@ -145,7 +145,7 @@ def main():
                 in the 'required-projects' parameter on the job.
                 To fix this issue, add:
 
-                  {src}
+                  {project}
 
                 to 'required-projects'.
 
@@ -153,9 +153,10 @@ def main():
                 is deprecated and this shim is only present for transition
                 purposes. Start thinking about how to rework job content to
                 just use the git repos that zuul will place into
-                /home/zuul/src directly.
+                {repo_src_dir} directly.
                 **********************************
-                """.format(src=src)))
+                """.format(
+                    src=src, project=project, repo_src_dir=REPO_SRC_DIR)))
             sys.exit(1)
 
         dst = dests[project]