Merge tag 'sparks-3.5.5' into develop

3.5.5
celery beat doesn't have -E argument.
auto-switch off env.parallel if there is only one machine in pick().
more worker names (I should stop creating them some day and make the whole thing completely dynamic…)
ability to define worker concurrency and max-tasks-per-child with sparks_role@hostname or celery_queue@hostname, added to the already existing possibilities.
implement the `remove` command to remove service configuration from workers, and some enhancements to service management (eg. don't try to do anything if service file is not installed).
Ability to pick `roles` like we can already pick hosts. Allow very fine (un-)deployment on a set of already running machines.
Bump version for 3.6
Merge branch 'release/3.6'