Simplify retargetable test framework

Simplify retargetable test framework

The retargetable testing prototype previously relied on each test case
defining the 'scenarios' attribute used to parametize testing with
testscenarios.  Anticipating the requirement to retrofit the imported
tempest api test cases, this change moves scenario definition to a
base class since scenarios are common across all api tests.

This change also sets the retargetable test to skip when invoked
against rest.  Tempest uses class-level setup for auth and this needs
to be broken out into fixtures before the retargetable testing will
work again.

Change-Id: I70eb21db9b983d45e9bcc7ea90e36f202d3e3e45