New way to override config variables

[Tim Armstrong <ta...@cloudera.com>]

Hi All,
  I just wanted to draw your attention to this change that went into
master: https://gerrit.cloudera.org/#/c/5545/

It fixes a lot of the problems with impala-config.sh that people have
experienced that manifest as errors downloading toolchain tarballs (we've
had at least one discussion on this list). That should stop happening.

The only thing that will stop working is that you can't set certain config
variables (mainly version numbers) by setting the environment variables
before sourcing impala-config.sh.

It adds a different way to override config variables locally (aside from
sticking them in your .bashrc). If you create a file called
bin/impala-config-local.sh, you can assign variables in there that will
override the defaults in impala-config.sh. E.g. you could add:

   IMPALA_HADOOP_VERSION=3.0.0-alpha1

This file is in .gitignore so should stay the same when you switch between
branches.

- Tim