add support for $update_channel to match coreos.com documentation

[busser]

These changes implement the $update_channel variable in config.rb.sample and Vagrantfile to match the coreos.com documentation.

Setting the $update_channel variable in config.rb to alpha, beta, or stable will make Vagrant use the corresponding box of the official CoreOS release.

There is no change to the project's default behavior ($update_channel is set to alpha by default).

This pull request is comparable to pull request #310.

[oliverkuntze]

hey mate,
i accidently included your patch in my pull request #317.
this is my first time using git. i apparently should have forked the repo, created a feature branch and than created a pull request for that branch.
since i didn't want to wait for your pull req to be accepted i merged it into my fork. apparently that's why it is now also in my pull request.
didn't mean to take your credit.
sorry for my mistake.

[lucab]

LGTM