## 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
````

Author: Mark Grover <mark@apache.org>

Closes #12250 from markgrover/spark-14477.