ConditionalApplyPlugin: Allow every apply to be optionally skipped

[kmanning]

As a developer, I want to be able to arbitrarily "deploy" to any environment that I want, so that I can fix problems in a emergency, and not be "slowed down" by the pipeline.

• In the master branch, there are 2 options - confirm that you want to apply, or abort
• What if we gave customers a third option - "skip".
• If the user chooses skip, then "apply" is not run for the current environment, and instead simply continues through the pipeline as though "apply" had run

[kmanning]

Example code:

Jenkinsfile.init(this)
ConditionalApplyPlugin.enableSkipApply()

def validate = new TerraformValidateStage()
def deployQa = new TerraformEnvironmentStage('qa')
def deployUat = new TerraformEnvironmentStage('uat')
def deployProd = new TerraformEnvironmentStage('prod')

// When prompted to confirm "apply qa", user now has a 3rd option to simply skip over, and continue to uat
validate.then(deployQa)
// When prompted to confirm "apply uat", user now has a 3rd option to simply skip over, and continue to prod
        .then(deployUat)
// When prompted to confirm "apply prod", user now has a 3rd option to simply skip over and finish the pipeline
        .then(deployProd)
        .build()