Support of context for benchmarks

Currently we have 3 context things:
1) UserGenerator context - which allows to create & delete users & tenants
2) Resource cleaner - which allows to delete resource that were created from users
3) SSH access prepare - which allows to connect via ssh to VMs

In future we will have probably even more context (ftp, some resource context and so on on)

The goal of this blueprint is to make support of "context" in common way without spaghetti in code + context should be configurable in two ways:
1) via config (e.g. create 400 images in glance before starting)
2) via decorator of benchmark scenario (in case of NovaServers.boot_runncomand_server() )
3) context object with all info should be passed to each scenario call.