Extract dist config handling, support {dist_node, ...}

[ferd]

This commit moves the handling of distribution config and starting out
of rebar_prv_shell and into rebar_dist_utils. The module is able to
handle standard config options and boot a distributed node mode. This
could be used in plugins (once it is exposed) and other providers like
CT.

Configuration is also expanded so that options like:

{dist_node, [{sname, atom()}, {name, atom()}, {setcookie, term()}]}

can be used and will be handled as a default. The config handler
supports similar terms from the command line being parsed in if the
calling provider supports them.

A test suite is added for configuration handling.

---

This is open for comments in how to handle the options or how to name them.

[tsloughter]

+1 Very nice. Maybe a more clear name for the config option would be dist_node?

[ferd]

Yeah I'm okay with that rename. Will do it right away.

[ferd]

Renamed.