scenario/network_basic_ops: detach floating-ip

test that a floating ip is successfuly disassociated from VM's port and
VM becomes unreachable

helper:
api/network/common.py
    override update() method for DeletableFloatingIp - handels
    neutron-update and updates self with result

manager:
    _disassociate_floating_ip(self, floating_ip) - use
    DeletableFloatingIp.update()
    _check_vm_connectivity(should_connect=True)
        add negative connectivity test option - will check ping
        only

Change-Id: Iab5f89d92bc2c4ebb9baff2dee4cade0a4cb07d6
Merge "scenario/network_basic_ops: detach floating-ip"