Better handling of namespace validation to fix a number of issues

- Fixes a bug which allows for underscores at the beginning of namespaces: Fixes #1849
- Allows dots and dashes for newer Docker clients: Fixes #1188
- Has the UI display better messaging associated with namespace entry

test/test_api_usage.py

@@ -551,7 +551,7 @@ class TestCreateNewUser(ApiTestCase):
                                            email='test@example.com'),
                                  expected_code=400)
 
-    self.assertEquals('Invalid username auserName: Username must match expression [a-z0-9_]+',
+    self.assertEquals('Invalid username auserName: Username must match expression ^([a-z0-9]+(?:[._-][a-z0-9]+)*)$',
                       json['detail'])
 
   def test_createuser(self):