doc openstack builder: region req'd only for rackspace

Modify openstack builder doc to say that specifying a region is only
required for public rackspace and is NOT required for a private
openstack cloud.

Update openstack builder doc to include private openstack example

This illustates how to use the OpenStack builder with a private
OpenStack cloud, as opposed to a public cloud such as Rackspace public
cloud.

/cc @andytson - Wanted to CC you in here so you could see some changes I
made.

First, I changed commit to opt-in, so you set "commit": true to get
commit behavior. This simplifies the logic a bit. Then, I removed the
skipping for StepExport and StepCommit and put that into the Builder
itself. This simplifies those steps (limits abstraction leakage).
Otherwise, everything looks great!

Merge branch 'feature/docker-image' of github.com:andytson/packer into andytson-feature/docker-image

Conflicts:
	config.go