Check a rule existence for a specific security group

Check a rule existence for a specific security group

    Implement a functionality to check if a rule concerning
    a specific security group exists
    - function get_security_group_rules(neutron_client, sg_id)
      returns the list of the security rules for a specific security group
    - function check_security_group_rules(neutron_client,
                                          sg_id, direction,
                                          protocol,
                                          port_min=None,
                                          port_max=None)
      checks if a specific rule for a specific security group exists
      and returns True or False
    - implement unit tests for the two new functions

     This new functionality is needed for sdnvpn project

Change-Id: Ib930bc9a76141932f4164d88e2640b49f3df4d77
Signed-off-by: tomsou <soth@intracom-telecom.com>