commont task to create lxc vm in separete file `prepare_lxc_host`,
avoid redundancy of statements in each vm-specific playbook file.

Playbooks updated to import `prepare_lxc_host`:
- ldap
- matrix
- nextcloud
- projects
- status

Add role prefix to role specifig variable to avoid conflicts when
using multiples role on the same machine on the same play.

`lxc-ssh.py` removed.

All Playbbooks now user `ssh_lxc` connection.

`ansible_ssh_lxc_name` variable used to specify container name.

Tested and worked correctly with `python==3.8.2` and `ansible==2.9.6` on the
controller and `python==2.7` on the target.