add generic contexts

[mickours]

To be independent from underlying interpreter (bash, python, ruby, powershell...), the context declaration variables like workdir should be remove and replace by an inline command like:

bash -c "cd /tmp && bash"

It will be a good idea to enable generic context definition linked to the exec_* command. For example:

# in recipe
global:
  context:
    - my_ctx: bash -c "cd /tmp && bash"

# in steps
- my_step:
  - exec_my_ctxt: echo "Hello :-)"