[SPARK-1776] Have Spark's SBT build read dependencies from Maven.

Patch introduces the new way of working also retaining the existing ways of doing things.

For example build instruction for yarn in maven is
`mvn -Pyarn -PHadoop2.2 clean package -DskipTests`
in sbt it can become 
`MAVEN_PROFILES="yarn, hadoop-2.2" sbt/sbt clean assembly`
Also supports
`sbt/sbt -Pyarn -Phadoop-2.2 -Dhadoop.version=2.2.0 clean assembly`


Mind adding a comment here like?

```
// Provides compatibility for older versions of the Spark build
```
The name is pretty straightforward but it might be good to make it crystal clear for developers