Making execution context immutable

* The idea of this commit is that mutable execution context
  doesn't make any sense in general because of variety of
  workflows we may have. So that, for example, if we have
  multiple parallel routes in a workflow graph then there
  can't be any shared context between them. There's only
  inbound and outbound context for a certain task.

  At the same time it turned to be convenient to leave
  immutable execution 'context' field that includes all
  the initial variables at the moment a workflow started
  (including OpenStack related data accessible via key
  'openstack' and information about  execution itself
  accessible via '__execution').

Change-Id: I0257a0952bb661a7ad2c44742584a02609c54fc3
Merge "Making execution context immutable"