Improve context storage to share data through whole run

[legnadev]

Improved the managing of storages:

• Added run_storage to store information during the whole test execution
• Storages are merged to be able to get the information always from context.storage / [CONTEXT:]
• In steps, be able to store values into desire storage by using [key], [FEATURE:key] and [RUN:key]. Example:
  • I store in key "store_key" the value "XXX"
  • I store in key "[FEATURE:store_key]" the value "XXX"
  • I store in key "[RUN:store_key]" the value "XXX"
• Implemented new method store_key_in_storageto manage it