Allow get_packager_acls to restrict the ACLs retrieved to a specific set of ACLs

The pkgdb2 API suppport restricting the ACL retrieved for a packager to a specific set of
ACLs, this commit allows the pkgdb2 module to support this.
Fix documentation for the options available for the acls kwarg of get_packager
Add tests for the get_packager method restricted to some ACLs only