Standard "Send an email" action

We need to have a standard Mistral action accessible in DSL for sending an email address. The suggested way of accessing it is "std:email" where "std" is the namespace for standard built-in actions that don't require an explicit declaration in DSL.

If a task is associated with this action then it should have the following parameters:
* "to" - a list of email addresses as a comma-separated list
* "subject" - email message subject
* "body" - email message body

The DSL snippet:

Workflow:
  tasks:
    sendCreateVMError:
      action: std:email
      parameters:
        to: <email address hidden>, <email address hidden>
        subject: Workflow error
        body: Failed to create a VM in execution {$.execution.id}
      on-finish: end