Add rbenv.install_rbenv as separate state

This add ability to install rbenv and required plugin ruby-build as a separate state. Previously, they got installed as part of installing a ruby. If you want to have plugins installed prior to installing a ruby, you can do that now by using the install state and require that in the state for each ruby install.

This can be used to customize the rbenv install a little more and is a necessary step to address #9300.

As an example state in rbenv.sls:
rbenv:
  rbenv.install_rbenv
ruby-2.0.0-p451:
  rbenv.installed:
  - require:
    - rbenv: rbenv