Add input scenarios for tempest scenario tests

The aim of the blueprint is to provide a facility for generating and applying input scenarios to the tempest scenario tests (see https://etherpad.openstack.org/p/icehouse-summit-qa-future-of-scenario-testing).
This is especially useful when running scenarios tests against real-life clouds, to avoid being constrained to the input values in tempest.conf.

Example of input scenarios are image_ref and flavor_ref.

The testcenarios library by Rob Collins provides exactly what we need.