Run docker-machine in parallel for better startup time

[dduportal]

Let's say for the set of worker (for managers, it can be problematic for the convergence of the internal KVP).

They are created/started/stopped sequentially. Running in parallel could be an interesting improvement :)

Some help there: https://www.codeword.xyz/2015/09/02/three-ways-to-script-processes-in-parallel/

I think that using xargs seems a good fit ?

[xinity]

i love the idea behind managing workers simultaneously (aside the IP issue that should also be dealt with).
i'd rather not use xargs which some drawbacks regarding parameters and exit code :(