Use :rvm_path config approriately instead of always defers to environment variable

Related to
- https://github.com/wayneeseguin/rvm/issues/1466
- https://github.com/wayneeseguin/rvm-gem/pull/11
- https://github.com/fnichol/chef-rvm/issues/159

This uses `rvm_path` config when possible instead of depending of `self.class. default_rvm_path` which afaik only works when rvm is sourced / `source_rvm_environment` is called ( which is not the case when `:rvm_by_path` is used  in chef-rvm)