Add Terraform installation to PATH and add @Symbol annotation

[michelzanini]

• Gradle 2.x has been deprecated and is no longer supported in IntelliJ
• Add Terraform installation to PATH and add @symbol annotation

[michelzanini]

This changes will allow people to use Terraform in pipelines as following:

pipeline {
 
  tools {
    terraform '0.12.21'
  }

  stage('Version') {
      steps {
        sh('terraform version')
      }
  }

}

This is possible by adding @Symbol annotation as terraform.
Also, to make this work correctly, the installed tool needs to be added to PATH so it will work.

[michelzanini]

@dpires can you please review this? Thanks.

[dpires]

Looks good.