[SPARK-938][doc] Add OpenStack Swift support

See compiled doc at 
http://people.apache.org/~rxin/tmp/openstack-swift/_site/storage-openstack-swift.html

This is based on #1010. Closes #1010.
minor - but could this say `conf/` rather than `/spark/conf`... we usually don't assume the name of the root folder (in many cases it e.g. has a version)