Rename validate_one_role to validate_one_for_role

This is clearer; we keep the old name for now.
Add validate_at_least_n_for_role method

This helps validate proposals when they can have several nodes for one
role.
Add ServiceObject.display_name API
Add validate_dep_proposal_is_active method
Add find_dep_proposal method

This is something that is duplicated in many barclamp, to find
dependencies.
Merge pull request #845 from vuntz/validation-api

Add some helpers to create/validate proposals