dyngroup: Refs #2240 Fix TB on package edition

When you edit a package, if there is a convergence group on this
package, this convergence group will be edited.

* For convergence group edition, we need name of parent group
* dyngroup.get_group() is called
* If current user doesn't have rights to see parent group (it's
  possible, he can edit a package, and this package is linked to
  group of other user, edition of convergence group will fail

So, get parent group as root fixes the problem.