Deduplicate the node:parameters and node:variables rake tasks
Deduplicate the nodegroup:parameters and nodegroup:variables rake tasks

See 554e679559454e2e3e1e538205d497b4cfcb8e77 for an important bit of
history about the parameters/variables parsing strategy.
Handle missing arguments for node:variables and node:delvariables
Simplify node:delvariables
Print rake errors to stderr
Merge pull request #300 from sodabrew/rake_cleanup

Rake cleanup