SPARK-4466: Provide support for publishing Scala 2.11 artifacts to Maven

The maven release plug-in does not have support for publishing two separate sets of artifacts for a single release. Because of the way that Scala 2.11 support in Spark works, we have to write some customized code to do this. The good news is that the Maven release API is just a thin wrapper on doing git commits and pushing artifacts to the HTTP API of Apache's Sonatype server and this might overall make our deployment easier to understand.

This was already used for the 1.2 snapshot, so I think it is working well. One other nice thing is this could be pretty easily extended to publish nightly snapshots.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #3332 from pwendell/releases and squashes the following commits:

2fedaed [Patrick Wendell] Automate the opening and closing of Sonatype repos
e2a24bb [Patrick Wendell] Fixing issue where we overrode non-spark version numbers
9df3a50 [Patrick Wendell] Adding TODO
1cc1749 [Patrick Wendell] Don't build the thriftserver for 2.11
933201a [Patrick Wendell] Make tagging of release commit eager
d0388a6 [Patrick Wendell] Support Scala 2.11 build
4f4dc62 [Patrick Wendell] Change to 2.11 should not be included when committing new patch
bf742e1 [Patrick Wendell] Minor fixes
ffa1df2 [Patrick Wendell] Adding a Scala 2.11 package to test it
9ac4381 [Patrick Wendell] Addressing TODO
b3105ff [Patrick Wendell] Removing commented out code
d906803 [Patrick Wendell] Small fix
3f4d985 [Patrick Wendell] More work
fcd54c2 [Patrick Wendell] Consolidating use of keys
df2af30 [Patrick Wendell] Changes to release stuff