@dylanfareed: Revise ow:logs task to accept a specific instance and fully qualified log path

@ibussieres helpfully added an `ow:logs` task to tail logs (https://github.com/artsy/momentum/pull/6).

That was apache/passenger-specific, so I tweaked the interface to accept a specific instance name (rather than layer, from which a random instance would be selected), and a fully qualified log path (rather than make assumptions about paths and file extensions).

This allows commands such as:

    bundle exec rake ow:logs[production,worker1,/srv/www/gravity/shared/log/delayed_job.log]

Or even:

    bundle exec rake ow:logs[production,rails-app3,/srv/www/gravity/shared/log/*.log]

If it gets tiresome to type long file paths, we can always create wrapping tasks that specify the commonly tailed logs for us.