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
Fix volume transfer tests with tenant isolation disabled

This commit fixes the volume transfer tests for when tenant isolation
is disabled. The volume transfer tests still had a reference to the old
auth style to get the tenant name if tenant isolation was disabled.
The recent auth_provider refactor didn't update this call so the test
would fail. This commit updates it to use the new credential dict to
get the tenant_name

Change-Id: Ie7fe39baf6f62fa6966631d7890891252602ed8d
Merge "Adds test_show_XX_fields cases to show some fields"
Merge "Fix volume transfer tests with tenant isolation disabled"
Merge branch 'master' of https://github.com/openstack/tempest