Fix setting invalid GOOGLE_APPLICATION_CREDENTIALS env variable

- set CLOUDSDK_CORE_ACCOUNT instead

Add computeFirewallRulesList step

[BREAKING] Rename steps

In order to be future-proof when more steps will be added the existing steps have been renamed:
createFirewallRule --> computeFirewallRulesCreate
deleteFirewallRule --> computeFirewallRulesDelete

General agreement is to name the steps as per gloud CLI documentation

Add deleteFirewallRule step

Throw exception from withGCP call in case auth is not successful

Otherwise the job will hang indefinitely

Make use of EnvVars in createFirewallRule step

So that it can be executed as:

withGCP(credentialsId: "CREDS") {
    createFirewallRule(..)
}

Add createFirewallRule step

Enabling CD (#3)