Started implementation of fix for additional vars

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
[FIXED JENKINS-19889] - Fixed handling of "Additional variables"
Now the plugin exposes "exported variables" from tool's global configuration. I've also added a stub for future tests.

Resolves https://issues.jenkins-ci.org/browse/JENKINS-19889

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
Merge pull request #12 from synopsys-arc-oss/additional_variables_fix

[FIXED JENKINS-19889] - Additional variables fix