Remove ec2-user from playbooks

Remove:

    remote_user: ec2-user

from a bunch of playbooks, because I'm using OpenStack and don't have
that user and it's not necessary anyway, because a default remote_user
is already set in src/ansible.cfg
Merge branch 'remove_ec2-user_from_playbooks' into generalize_so_not_quite_so_specific_to_ec2