Do not report docker kill/rm attempts as errors

These are just defensive cleanup tasks that we run.
In the good case, there's nothing to kill or remove, so they trigger an
error like this:

> Error response from daemon: Cannot kill container: something: No such container: something

and:

> Error: No such container: something

People often ask us if this is a problem, so instead of always having to
answer with "no, this is to be expected", we'd rather eliminate it now
and make logs cleaner.

In the event that:
- a container is really stuck and needs cleanup using kill/rm
- and cleanup fails, and we fail to report it because of error
suppression (`2>/dev/null`)

.. we'd still get an error when launching ("container name already in use .."),
so it shouldn't be too hard to investigate.

roles/matrix-base/defaults/main.yml

@@ -66,6 +66,7 @@ matrix_host_command_chown: "/usr/bin/env chown"
 matrix_host_command_fusermount: "/usr/bin/env fusermount"
 matrix_host_command_openssl: "/usr/bin/env openssl"
 matrix_host_command_systemctl: "/usr/bin/env systemctl"
+matrix_host_command_sh: "/usr/bin/env sh"
 
 matrix_ntpd_package: "ntp"
 matrix_ntpd_service: "{{ 'ntpd' if ansible_os_family == 'RedHat' or ansible_distribution == 'Archlinux' else 'ntp' }}"