(GH-30) Allow :node meta-parameter to accept a string

For convenience, the :node meta-parameter now accepts the string of the node
name (from .nodeset.yml) instead of just an RSpecSystem::Node object.

This is more to improve the multi-node usability.

Signed-off-by: Ken Barber <ken@bob.sh>
(GH-30) Create a documentation section on multi-node support

Signed-off-by: Ken Barber <ken@bob.sh>
(GH-30) Simplify test for sles

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #41 from kbarber/ticket/master/GH-30-multi-node-docs

Improve multi-node docs & usability