Add an initial stab at docstrings - enable docstring pylinter

tests.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 #
 # -*- coding: utf-8 -*-
+""" This is a unit test suite for vppcfg """
 
 import sys
 import glob
@@ -36,6 +37,8 @@ def example_validator(_yaml):
 
 
 class YAMLTest(unittest.TestCase):
+    """This test suite takes a YAML configuration file and holds it against the syntax
+    (Yamale) and semantic validators, returning errors in case of validation failures."""
     def __init__(self, testName, yaml_filename, yaml_schema):
         # calling the super class init varies for different python versions.  This works for 2.7
         super().__init__(testName)
@@ -43,6 +46,7 @@ class YAMLTest(unittest.TestCase):
         self.yaml_schema = yaml_schema
 
     def test_yaml(self):
+        """ The test executor """
         test = None
         cfg = None
         ncount = 0
@@ -103,6 +107,7 @@ class YAMLTest(unittest.TestCase):
 
 
 if __name__ == "__main__":
+    """ The main program """
     parser = argparse.ArgumentParser(formatter_class=argparse.RawTextHelpFormatter)
     parser.add_argument(
         "-t",