Add support to execute workflows on arbitrary clouds

Mistral is configured to execute workflows on a specific cloud (VIM (Virtual
Infrastructure Manager) in ETSI MANO terms). In order to execute workflows on a
different cloud one needs to start a new Mistral instance with the cloud
specific configuration. When Mistral is used in standalone mode, this is an
undesired overhead and can cause unpredictable resource usage patterns.

This blueprint proposes to extend the workflow execution parameters to make it
possible to target a specific cloud without modifying the configuration of the
Mistral service.

rakhmerov: Just an addition. This is also needed for scenarios when we want to
manage/orchestrate activities happening in different clouds, aka cross-cloud
orchestration.