Adds test_show_XX_fields cases to show some fields

This patch modifies the _show method to make it accept field_list.
It adds the cases to show some fields for network/subnet/port.
A related url example is
/v2.0/networks/103d47a3-XXXX-018c095ab35d?fields=id&fields=name

Change-Id: If2b9edd67ad0c43679332ec28ca80940c0c67d9e
Merge "Adds test_show_XX_fields cases to show some fields"