Issue 331: terraform-pipeline should not fail if DockerPipeline plugin not installed

[kmanning]

• Issue Defect: If Docker Pipeline plugin is not installed, terraform-pipeline cannot load #331
• How to reproduce:
  • Have a jenkins instance that does not have Docker Pipeline plugin installed
  • Load a terraform-pipeline project in Jenkins, and load the library with Jenkinsfile.init(this)
  • Expected: the pipeline loads with no error
  • Actual: the pipeline errors with:

hudson.remoting.ProxyException: groovy.lang.MissingPropertyException: No such property: docker for class: WorkflowScript
	at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:53)
	at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.getProperty(ScriptBytecodeAdapter.java:458)
	at com.cloudbees.groovy.cps.sandbox.DefaultInvoker.getProperty(DefaultInvoker.java:39)
	at com.cloudbees.groovy.cps.impl.PropertyAccessBlock.rawGet(PropertyAccessBlock.java:20)
	at Jenkinsfile.init(Jenkinsfile.groovy:59)
	at Jenkinsfile.init(Jenkinsfile.groovy)

• Jenkinsfile constructor captures the WorkflowScript's docker value. This value only exists if the DockerPipeline is not installed. If it's not installed, terraform-pipeline cannot function (even if you're not using docker features), because Jenkinsfile.init(this) fails.
• The docker reference in Jenkinsfile is actually dead code - remove it. This fixes the failing Jenkinsfile.init(this) if DockerPIpeline plugin is not installed.