Swarm slave must be provisioned with dynamic env properties

The swarm slave currently embeds default environment properties, leading to collisions between containers.

The swarm slave must benefit of the same provisioning as the standard former slave. For this, it has to

• be called with MULTIDB property, when relevant
• source /etc/environment

Document /etc/environment and /etc/_my_init.d/10_setup_environment.sh

(source: https://github.com/nuxeo/nuxeo-ansible-jenkins/blob/master/roles/slave_tools/tasks/main.yml#L347)

The issue seems to come from the default overriding setup by the swarm client installer. If so, update it to sample and document usage but with no default values.