build: fix python load paths

Fixed python load paths so that vppcfg will work installed as python
library and standalone from the source directory, fixing load
pathes for resources such as yaml files along the way.

Added a make target for pylint called 'make check-style', fixed a
number of minor pylint issues along the way.

Signed-off-by: Ray Kinsella <mdr@ashroe.eu>

vppcfg/config/test_interface.py

@@ -15,12 +15,13 @@
 """ Unit tests for interfaces """
 import unittest
 import yaml
-import config.interface as interface
+from . import interface
+from .unittestyaml import UnitTestYaml
 
 
 class TestInterfaceMethods(unittest.TestCase):
     def setUp(self):
-        with open("unittest/test_interface.yaml", "r") as f:
+        with UnitTestYaml("test_interface.yaml") as f:
             self.cfg = yaml.load(f, Loader=yaml.FullLoader)
 
     def test_enumerators(self):