add timeout-var option to the timeout module

jenkins build-timeout plugin version 1.13 has a new option[1] to expose the
timeout value via an environment variable.  Adding this option to the
equivalent jjb module.  Also created tests for the examples.

[1] https://github.com/jenkinsci/build-timeout-plugin/pull/22

Partial-Bug: #1287883
Change-Id: Ia0d6570e9aae55ceadf076716fc8ad7688ff6214
Merge "add timeout-var option to the timeout module"