Allow subclasses of task.Ansible to provide an inventory group name

Allow subclasses of task.Ansible to provide an inventory group name

Subclasses of task.Ansible can now provide an inventory group name to
assign all nodes to when creating a dynamic inventory.

This PR also makes ansible.CephLab use the group 'testnodes'. Without this,
teuthology can not run the testnodes.yml playbook against it's locked nodes
using a dynamic inventory as it's restricted to the testnodes group.

Signed-off-by: Andrew Schoen <aschoen@redhat.com>