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
Drop old deprecated validation API

It got only used in keystone (which is not using it anymore) and was
exposing in the core of crowbar details about the database barclamp
(which is not core).
Merge pull request #846 from vuntz/drop-old-api

Drop old deprecated validation API