ε-BAL: add support to inter-domain workflows

[ccicconetti]

This requires the ε-BAL to enter the picture.

Required changes include:

1. Controller data structures must take into account the presence of ε-BAL instances in the orchestration domains
2. The PatchRequest message has to be modified to also include the end-points of balancers.
3. The ε-BAL has to be implemented.
4. Possible modifications may be needed to the callback tables in the functions.