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

[mattbrictson]

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.

[will-in-wi]

Should the ticket for a more generalization solution be on the capistrano project?