Modified simplenode.runner to start from alternative directory

[uwiger]

This patch contains two additions to simplenode.runner:

1. Check if vm.args exists in CWD; if so, use it. This makes it
   easier to start multiple concurrent nodes on a single machine
   from one rebar-created release (starting each node from its
   own directory, with its own copy of vm.args and e.g.
   sys.config, log directory, database directory, etc.)

2. Add the targets start_boot <file> and console_boot <file>.
   This is used to select a different boot script. The 'setup'
   application (http://github.com/esl/setup) builds a special boot
   script for installation (all apps loaded but not started,
   making it possible to run install hooks with the full code path
   in place).

[uwiger]

An example of how we make use of this when preparing and starting a node:
https://gist.github.com/2365439

[joewilliams]

+1 Looks like this shouldn't effect existing behavior and is a good addition.

[uwiger]

Clarified the description in the original pull request message, and also amended the commit message.

[ghost]

+1.

[ghost]

Thanks, merged.