CIVICRM-273 Volunteer Roster does not show admin-only opportunities in backend

When, as an admin, you go to view the Volunteer roster on a project, any non-public roles are not shown with the roster:

volunteer_roster_no_roles.png

This is due to the _get_needs function in CRM/Volunteer/BAO/Project.php using the VolunteerNeed.get api with the visibility_id set to public.

For the site we found this on, we've worked around this issue by patching the call in question to allow either public or admin visibility, but I'm unsure of where else this function is exposed.

Proposed solution is to provide an optional parameter to the _get_needs function that defaults to filtering by public roles only so that it can be controlled from closer to the view.

Agileware reference CIVICRM-273