Add support for environment variables in source and class text fields in the plugin's configuration

[NonGrate]

I've met the problem, that when environment variables are used in the JaCoCo configuration in Jenkins.

Exclusions variables is parsed and the value is sent to the JaCoCo report generation, but source and classes paths are passed to the report generation as strings.

This pull request fixes the bug and allows to use environment variables in the class paths and source paths test fields.

Here's the opened issue: https://issues.jenkins-ci.org/browse/JENKINS-36168

[NonGrate]

Haven't wrote test yet. Planning to do that.

[don-vip]

@NonGrate are you still planning to finish the PR? I'd be glad to help.