Make resource status checking letter case independent

Recently we've faced the fact that while in Nova you should check the readiness
of a resource by its "AVAILABLE" status, in Cinder that should be written in
lower case, i.e. "available". With this patch there will be no more worries
about such details for benchmark scenario writers.

Blueprint benchmark-engine-refactoring

Change-Id: Ie93104b74ac25de46a1d2a5f0d23a3131aeecffa