TerraformPlugin: make it easy to detect the current terraform version

[kmanning]

UseCase: I want to be able to easily detect the correct terraform version, so that I can use that version for other purposes - like pulling in a dockerfile image that automatically matches the version of terraform that I'm using.

The example below pulls a terraform docker image that matches the version of terraform that was detected for your project.

Jenkinsfile.init(this)

def terraformVersion = TerraformPlugin.checkVersion()
AgentNodePlugin.withAgentDockerImage("hashicorp/terraform:${terraformVersion}")
               .withAgentDockerImageOptions("--entrypoint=''")
               .init()
...