Add support for properties in junit xml.

jenkins junit xsd supports `<properties>` under `<testsuite>`.
This change allows the plugin to be configured to load those properties from
a json file (dictionary with key-value pairs); which you have to opt-in for by
specifying `test_properties` in the configuration.
Merge pull request #222 from dnozay/junitprop

add support for properties in junit xml.