Update 'edx' group to 'adm' group in LMS setup

* Use the 'adm' group for permissioning assets needed during LMS
  initialization, because ubuntu doesn't have the edx user until it logs
  out and logs back in.

This is needed, @jarv, because of the VPC stuff. When you use the paramiko transport, ansible logs in and then out for every play. But to run ansible through a bastion to a VPC subnet, you have to use the SSH transport with a forward, which establishes a single connection and uses it the whole time.  So for soup-to-nuts host bringup in one playbook, ubuntu doesn't have a way to get its new group membership. So we use a group that ubuntu already belongs to instead.