[SPARK-16671][CORE][SQL] Consolidate code to do variable substitution.

Both core and sql have slightly different code that does variable substitution
of config values. This change refactors that code and encapsulates the logic
of reading config values and expading variables in a new helper class, which
can be configured so that both core and sql can use it without losing existing
functionality, and allows for easier testing and makes it easier to add more
features in the future.

Tested with existing and new unit tests, and by running spark-shell with
some configs referencing variables and making sure it behaved as expected.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #14468 from vanzin/SPARK-16671.