Added option to specify applications to start on cluster nodes

[pmenhart]

• Default behaviour: start_nodes/3 starts all applications that are loaded on the master node. There are situations where some apps are not desirable on slave nodes, or where their start order is causing problems.
• New option :app_names accepts an ordered list of applications to be started. Very useful for test scenarios where only a subset of apps should be copied over, or where the order of applications starting is critical.
• Note that each application is started via Application.ensure_all_started/2, i.e. with all dependencies.

[pmenhart]

As a side effect, this PR solves also concern of PR #14 "Ensure all configuration merged before starts loaded applications in child nodes".

[whitfin]

I've merged this in the meantime, since the naming doesn't block the merge. I'm likely going to tweak the option to :applications rather than :app_names, but the rest looks good!