[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.

Author: Reynold Xin <rxin@apache.org>
Author: Gil Vernik <gilv@il.ibm.com>

Closes #2298 from rxin/openstack-swift and squashes the following commits:

ff4e394 [Reynold Xin] Two minor comments from Patrick.
279f6de [Reynold Xin] core-sites -> core-site
dfb8fea [Reynold Xin] Updated based on Gil's suggestion.
846f5cb [Reynold Xin] Added a link from overview page.
0447c9f [Reynold Xin] Removed sample code.
e9c3761 [Reynold Xin] Merge pull request #1010 from gilv/master
9233fef [Gil Vernik] Fixed typos
6994827 [Gil Vernik] Merge pull request #1 from rxin/openstack
ac0679e [Reynold Xin] Fixed an unclosed tr.
47ce99d [Reynold Xin] Merge branch 'master' into openstack
cca7192 [Gil Vernik] Removed white spases from pom.xml
99f095d [Reynold Xin] Pending openstack changes.
eb22295 [Reynold Xin] Merge pull request #1010 from gilv/master
39a9737 [Gil Vernik] Spark integration with Openstack Swift
c977658 [Gil Vernik] Merge branch 'master' of https://github.com/gilv/spark
2aba763 [Gil Vernik] Fix to docs/openstack-integration.md
9b625b5 [Gil Vernik] Merge branch 'master' of https://github.com/gilv/spark
eff538d [Gil Vernik] SPARK-938 - Openstack Swift object storage support
ce483d7 [Gil Vernik] SPARK-938 - Openstack Swift object storage support
b6c37ef [Gil Vernik] Openstack Swift support
(cherry picked from commit eddfeddac19870fc265ef406d87e1c3db9b54249)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>