[SPARK-14477][BUILD] Allow custom mirrors for downloading artifacts in build/mvn

[markgrover]

What changes were proposed in this pull request?

Allows to override locations for downloading Apache and Typesafe artifacts in build/mvn script.

How was this patch tested?

By running script like

# Remove all previously downloaded artifacts
rm -rf build/apache-maven*
rm -rf build/zinc-*
rm -rf build/scala-*

# Make sure path is clean and doesn't contain mvn, for example.
...

# Run a command without setting anything and make sure it succeeds
build/mvn -Pyarn -Phadoop-2.4 -Dhadoop.version=2.6.0 package

# Run a command setting the default location as mirror and make sure it succeeds
APACHE_MIRROR=http://mirror.infra.cloudera.com/apache/ build/mvn -Pyarn -Phadoop-2.4 -Dhadoop.version=2.6.0 package

# Do the same without the trailing slash this time and make sure it succeeds
APACHE_MIRROR=http://mirror.infra.cloudera.com/apache build/mvn -Pyarn -Phadoop-2.4 -Dhadoop.version=2.6.0 package

# Do it with a bad URL and make sure it fails
APACHE_MIRROR=xyz build/mvn -Pyarn -Phadoop-2.4 -Dhadoop.version=2.6.0 package

[markgrover]

I didn't use a trailing slash in the definition of the default value here since I'd rather have 2 slashes at the end (if the user did define it) instead of no slash.

[vanzin]

This is unrelated to your change, but I wonder if Spark really needs to download the Scala distribution like this, or if the maven/ivy dependency resolution is enough.

[vanzin]

LGTM.

[SparkQA]

Test build #55284 has finished for PR 12250 at commit fcbcf26.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[vanzin]

Unrelated test failure. Merging to master.