[SPARK-1812] Enable cross build for scala 2.11.

Was just trying out akka 2.3.3 (theoretically it should all work.)
I won't claim to be a maven-archetype-plugin expert, or to have figured out everything that we would need to do to start using archetyped builds (e.g. I haven't got sub-project builds with archetypes figured out yet), but the basics of archetypes are pretty simple to use and do get us a significant part of the way to cross-building Spark.

To see how the rudiments work, you should be able to quickly and easily do the following:

1) Clone the spark repo
2) cd to spark/core
3) mvn archetype:create-from-project
4) cd into the new target/generated-sources/archetype
5) mvn install
6) make a tmp dir someplace and cd into it
7) mvn archetype:generate -DarchetypeCatalog=local -DgroupId=org.apache.spark -DartifactId=spark-core_2.11 -Dversion=2.0.0-SNAPSHOT
8) select '1', the only archetype that you now have installed locally
9) observe that you now have a spark-core tree ready to build spark-core_2.11-2.0.0-SNAPSHOT (except for the spark-parent reference -- I told you, I haven't got sub-projects figured out yet.)