Add testcases for add/remove vNIC functions

Add testcases for add/remove vNIC functions:
test_attach_interface_with_port_id_existed
test_attach_interface_with_ipAddress_network_id
test_attach_interface_with_bad_request_exception_raised
test_detach_interface
test_detach_interface_with_bad_request_exception_raised

Change-Id: I8c21cb70fac446cac415b5a793c1213d6c548f7b
Closes-Bug: 1379251
Merge "Add testcases for add/remove vNIC functions"