Add support for requirements.txt.

Change-Id: I80874548a5e1391055150752f18ee9f26c8012ed
Reviewed-on: https://review.openstack.org/18891
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins

jeepyb/cmd/run_mirror.py

@@ -78,7 +78,9 @@ def main():
             run_command("git clean -x -f -d -q")
             print("*********************")
             print("Fetching pip requires for %s:%s" % (project, branch))
-            for requires_file in ("tools/pip-requires",
+            for requires_file in ("requirements.txt",
+                                  "test-requirements.txt",
+                                  "tools/pip-requires",
                                   "tools/test-requires"):
                 if os.path.exists(requires_file):
                     stanza = "-r %s" % requires_file