Fix callback registry notification for security group rule

Fix callback registry notification for security group rule

Some housekeeping was done in
 - SecurityGroupDbMixin:
   - create_rule_bulk() calls to create_rule();
   - registry notification is in create_rule();
   - separate validation for a single rule and for a group of rules
 - SecurityGroupServerRpcMixin:
   - overriden methods call to corresponding super class methods;

Hopefully code is now self-documented enough

Closes-Bug: #1461024
Change-Id: Ia75d7e206716bbe74aae89e4cebd0c2c40af68a8