Fix clearing allowed-address-pairs with port_update

[aswadrangnekar]

By default port-update with action=clear sends None
instead of empty list. This caused error. Used
convert_to in attribute map to convert received
None type to empty list in order to fix this error.

Closes-rally-bug: DE1788

[aswadrangnekar]

Test results below

---

neutron.tests.unit.test_extension_allowedaddresspairs.TestAllowedAddressPairs.test_update_add_address_pairs 0.347
neutron.tests.unit.test_extension_allowedaddresspairs.TestAllowedAddressPairs.test_create_address_gets_port_mac 0.330
neutron.tests.unit.test_extension_allowedaddresspairs.TestAllowedAddressPairs.test_update_fixed_ip_to_address_pair_ip_fail 0.319
neutron.tests.unit.test_extension_allowedaddresspairs.TestAllowedAddressPairs.test_update_address_pair_to_match_fixed_ip_and_mac 0.316
neutron.tests.unit.test_extension_allowedaddresspairs.TestAllowedAddressPairs.test_create_port_bad_ip 0.311
neutron.tests.unit.test_extension_allowedaddresspairs.TestAllowedAddressPairs.test_update_fixed_ip_to_address_pair_with_mac_fail 0.290
neutron.tests.unit.test_extension_allowedaddresspairs.TestAllowedAddressPairs.test_update_clear_routes_with_None 0.249
neutron.tests.unit.test_extension_allowedaddresspairs.TestAllowedAddressPairs.test_create_port_remove_allowed_address_pairs 0.236
neutron.tests.unit.test_extension_allowedaddresspairs.TestAllowedAddressPairs.test_create_port_allowed_address_pairs 0.226
neutron.tests.unit.test_extension_allowedaddresspairs.TestAllowedAddressPairs.test_equal_to_max_allowed_address_pair 0.215

---

Ran 18 tests in 11.360s

OK (skipped=3)