Add test for buildbot configuration passing flake8

This test requires the flake8 python module; to ensure it is installed,
add a requirements.txt file. Travis-CI will automatically install the
dependencies specified in that file.

requirements.txt

@@ -0,0 +1,3 @@
+# Python modules needed for testing
+# (Travis-CI will auto-install them from this file)
+flake8 == 2.5.4

tests/sls/buildbot/master/config_lint.py

@@ -0,0 +1,20 @@
+import os
+import subprocess
+
+from tests.util import Failure, Success, project_path
+
+
+def run():
+    CONF_DIR = os.path.join(project_path(),
+                            'buildbot', 'master', 'files', 'config')
+    # Have to specify master.cfg separately because it is not a .py file
+    command = ['flake8', CONF_DIR, os.path.join(CONF_DIR, 'master.cfg')]
+    ret = subprocess.run(command,
+                         stdout=subprocess.PIPE,
+                         stderr=subprocess.PIPE,
+                         universal_newlines=True)
+
+    if ret.returncode == 0:
+        return Success("Buildbot master config passed linting")
+    else:
+        return Failure("Buildbot master config lint check failed:", ret.stdout)