SPARK-3962 Marked scope as provided for external projects.

Somehow maven shade plugin is set in infinite loop of creating effective pom.

Author: Prashant Sharma <prashant.s@imaginea.com>
Author: Prashant Sharma <scrapcodes@gmail.com>

Closes #2959 from ScrapCodes/SPARK-3962/scope-provided and squashes the following commits:

994d1d3 [Prashant Sharma] Fixed failing flume tests
270b4fb [Prashant Sharma] Removed most of the unused code.
bb3bbfd [Prashant Sharma] SPARK-3962 Marked scope as provided for external.
[DOC][PySpark][Streaming] Fix docstring for sphinx

This commit should be merged for 1.2 release.
cc tdas

Author: Ken Takagiwa <ugw.gi.world@gmail.com>

Closes #3311 from giwa/patch-3 and squashes the following commits:

ab474a8 [Ken Takagiwa] [DOC][PySpark][Streaming] Fix docstring for sphinx
[SPARK-4429][BUILD] Build for Scala 2.11 using sbt fails.

I tried to build for Scala 2.11 using sbt with the following command:

```
$ sbt/sbt -Dscala-2.11 assembly
```

but it ends with the following error messages:

```
[error] (streaming-kafka/*:update) sbt.ResolveException: unresolved dependency: org.apache.kafka#kafka_2.11;0.8.0: not found
[error] (catalyst/*:update) sbt.ResolveException: unresolved dependency: org.scalamacros#quasiquotes_2.11;2.0.1: not found
```

The reason is:
If system property `-Dscala-2.11` (without value) was set, `SparkBuild.scala` adds `scala-2.11` profile, but also `sbt-pom-reader` activates `scala-2.10` profile instead of `scala-2.11` profile because the activator `PropertyProfileActivator` used by `sbt-pom-reader` internally checks if the property value is empty or not.

The value is set to non-empty value, then no need to add profiles in `SparkBuild.scala` because `sbt-pom-reader` can handle as expected.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #3342 from ueshin/issues/SPARK-4429 and squashes the following commits:

14d86e8 [Takuya UESHIN] Add a comment.
4eef52b [Takuya UESHIN] Remove unneeded condition.
ce98d0f [Takuya UESHIN] Set non-empty value to system property "scala-2.11" if the property exists instead of adding profile.