Multi-threaded hostname updates

[Sharpie]

Syncing /etc/hosts in a serial fashion across a dozen computers in remote clouds can take a good amount of time. Since the problem is highly parallel, we should investigate spawning a few worker threads to knock it out faster.

[Sharpie]

Looks like Vagrant 1.2.0 introduced a batch method on the Environment class which handles parallel execution of machine actions.

[Sharpie]

...and that method refuses to run anything in parallel if the provider backing a machine doesn't opt into it. Since we're just opening a SSH or WinRM connection, we hopefully don't need the same level of guarantees that arbitrary actions require.