Merge tag 'sparks-2.8.1' into develop

2.8.1
Don't deploy everywhere if user speficied an host or a list of hosts.
Revamp our `execute_or_not()` function to be able to run tasks on individual hosts and still having handy the sparks role(s) they belong to. Workaround Fabric reseting `env.host_string.role` (which is normal in this case and given the Fabric context).
(feature) Allow worker concurrency configuration, and (bug) insert concurrency only for workers (eg. not flower…).
bump version in __init__.py
Merge branch 'release/2.8.2'