rbenv improvements / brainstorm

[cwolferh]

A few comments about the way rbenv is implemented in this script:

1. It edits the user's .bashrc -- would prefer to have the user do
   that themselves (especially if they are already rvm users, they need
   to be aware that their path needs to be set correctly).

2. The check for "$HOME/.rbenv/version" would only succeed if the user
   had called "rbenv global" to set this file-- not necessarily a fair
   assumption.

3. Re-running the script causes the RUBY_VERSION to be re-downloaded
   and re-installed (for reference: aeolus-incubator/dev-tools does not)

If dev-tools and deltacloud-devtools are going to live on, they
probably should share a common bash script/function that sets up rbenv
and/or a given rbenv version. I had been hesitating taking that
approach, because it is nice to have everything in one script... but I
think the time has come. (Aside: a common function to wrap yum/apt
installs would also be nice)

Finally, I think aeolus-inclubator/dev-tools actually handles
installing rbenv and a particular ruby version (if not already
installed), setting "rbenv local" and "gem install bundler" all within
a block very well, but the fact that it is root running those commands
as $DEV_USERNAME makes it a bit of an eye sore and needs to be changed
(and would be with
https://github.com/aeolus-incubator/dev-tools/issues/ ).