Workflow execution environment

We need to have a notion of environment in Mistral. The idea is simple:
* Using Mistral API we can create a new environment containing a number of variables
* When start a workflow we can optionally specify an environment that must be used during workflow execution to fullfill certain variables accessed, for instance, as $.env.my_var.
* All these variables should be passed all the way through down to task actions so that we don't have to specify action parameters explicitly over and over again.