[SPARK-17058][BUILD] Add maven snapshots-and-staging profile to build/test...

[SPARK-17058][BUILD] Add maven snapshots-and-staging profile to build/test against staging artifacts

## What changes were proposed in this pull request?

Adds a `snapshots-and-staging profile` so that  RCs of projects like Hadoop and HBase can be used in developer-only build and test runs. There's a comment above the profile telling people not to use this in production.

There's no attempt to do the same for SBT, as Ivy is different.
## How was this patch tested?

Tested by building against the Hadoop 2.7.3 RC 1 JARs

without the profile (and without any local copy of the 2.7.3 artifacts), the build failed

```
mvn install -DskipTests -Pyarn,hadoop-2.7,hive -Dhadoop.version=2.7.3

...

[INFO] ------------------------------------------------------------------------
[INFO] Building Spark Project Launcher 2.1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
Downloading: https://repo1.maven.org/maven2/org/apache/hadoop/hadoop-client/2.7.3/hadoop-client-2.7.3.pom
[WARNING] The POM for org.apache.hadoop:hadoop-client:jar:2.7.3 is missing, no dependency information available
Downloading: https://repo1.maven.org/maven2/org/apache/hadoop/hadoop-client/2.7.3/hadoop-client-2.7.3.jar


[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] Spark Project Parent POM ........................... SUCCESS [  4.482 s]
[INFO] Spark Project Tags ................................. SUCCESS [ 17.402 s]
[INFO] Spark Project Sketch ............................... SUCCESS [ 11.252 s]
[INFO] Spark Project Networking ........................... SUCCESS [ 13.458 s]
[INFO] Spark Project Shuffle Streaming Service ............ SUCCESS [  9.043 s]
[INFO] Spark Project Unsafe ............................... SUCCESS [ 16.027 s]
[INFO] Spark Project Launcher ............................. FAILURE [  1.653 s]
[INFO] Spark Project Core ................................. SKIPPED
...
```

With the profile, the build completed

```
mvn install -DskipTests -Pyarn,hadoop-2.7,hive,snapshots-and-staging -Dhadoop.version=2.7.3
```

Author: Steve Loughran <stevel@apache.org>

Closes #14646 from steveloughran/stevel/SPARK-17058-support-asf-snapshots.

(cherry picked from commit 37d95227)
Signed-off-by: Reynold Xin <rxin@databricks.com>