Don't use 'become' in CI tests

Kolla Ansible now claims [1] to support executing as a user other than
root.  We should ensure that this is tested in CI.

This change removes the 'become' from hosts in the inventory, and sets
the remote user to 'kolla', as configured via the bootstrap-servers
command. The bootstrap-servers command and other ansible commands
executed before it still need to execute as the zuul user and not as
kolla, since kolla does not exist yet.

The autogenerated SSH private key in passwords.yml is now added to the
zuul user's SSH config, such that it can SSH as the kolla user, which
has authorised this key.

[1]
https://blueprints.launchpad.net/kolla-ansible/+spec/ansible-specific-task-become

Change-Id: I8f3587e2908bc5b8889cd6fbc01981a6186d33e6