Make config.get_boolean return given default value

Returning a default value (presumably a boolean) in case the configuration
option is not set, helps to implement opt-in / opt-out options that should
be false / true, unless explicitly set otherwise.

closes #3352
https://pulp.plan.io/issues/3352

AUTHORS

@@ -19,6 +19,7 @@ Patrick Creech (pcreech@redhat.com)
 David Davis (daviddavis@redhat.com)
 Michael DeHaan (mdehaan@redhat.com)
 Elijah DeLee (kdelee@redhat.com)
+Matthias Dellweg (dellweg@atix.de)
 Jay Dobies (jason.dobies@redhat.com)
 Filip Dobrovolny (fdobrovo@redhat.com)
 Máirín Duffy (duffy@redhat.com)

server/pulp/plugins/config.py

@@ -86,11 +86,11 @@ def get(self, key, default=None):
 
         return default
 
-    def get_boolean(self, key):
+    def get_boolean(self, key, default=None):
         """
         Parses the given key as a boolean value. If the key is not present or
-        is not one of the acceptable values for representing a boolean, None
-        is returned.
+        is not one of the acceptable values for representing a boolean, a default
+        value (defaulting to None) is returned.
 
         :param key: key to look up in the configuration
         :type  key: str
@@ -112,7 +112,7 @@ def get_boolean(self, key):
                 return True
             elif str_bool == 'false':
                 return False
-        return None
+        return default
 
     def flatten(self):
         """