Feature: rate limit for bundle:install task when using lots of servers

As mentioned by @gondalez in #93, the `bundle:install` task uses the default parallel SSHKit execution mode. This means if you have 100 servers you will potentially get 100 instances of `bundle:install` running concurrently. If a Gemfile uses a private gem server, this can overwhelm that server or run up against rate limits.

It would be nice to be able to configure the maximum parallelization of the `bundle:install` task.

However, I'd rather not introduce a `bundle:install`-specific configuration variable, because this sets a precedent for setting these sorts of variables for all kinds of Capistrano tasks (e.g. `git:clone/update`) that can run up against rate limits on shared resources. Ideally we could come up with a more generalizable solution.

Start by reading issue #93 and the implementation of the `bundle:install` task to understand how parallel SSHKit execution is selected. A complete solution would provide a general way to configure parallelization for tasks that may hit shared-resource rate limits, without adding a task-specific setting.

Written by the indexing model from the issue text.