Add server config option controlling certificate client auth

This change adds a server configuration option to control the
enforcement of TLS certificate client authentication. Before,
client TLS certificates had to include the extended key usage
extension with the clientAuth bit set to be used as sources of
client identity. The new configuration option,
enable_tls_client_auth, allows server admins to enable/disable
this requirement. The configuration setting is optional and the
server defaults to the original enforcing behavior if it is not
set. Admins must explicitly set the option to False to disable
enforcement.

kmip/tests/unit/services/server/test_server.py

@@ -119,7 +119,8 @@ class TestKmipServer(testtools.TestCase):
             '/etc/pykmip/certs/server.key',
             '/etc/pykmip/certs/ca.crt',
             'Basic',
-            '/etc/pykmip/policies'
+            '/etc/pykmip/policies',
+            False
         )
 
         s.config.load_settings.assert_called_with('/etc/pykmip/server.conf')
@@ -142,6 +143,10 @@ class TestKmipServer(testtools.TestCase):
             'policy_path',
             '/etc/pykmip/policies'
         )
+        s.config.set_setting.assert_any_call(
+            'enable_tls_client_auth',
+            False
+        )
 
         # Test that an attempt is made to instantiate the TLS 1.2 auth suite
         s = server.KmipServer(