Created the keep-old-app flag

[adamsiemion]

I have created a new arg flag keep-old-app, which skips deleting of the old app.

This was required by the project which currently work on.
We use Pivotal Cloud Foundry (PCF) container-to-container networking and Eureka service registry.

The way we use autopilot is the following (let's assume the service we are redeploying is called test-service:

• Eureka has our test-service already registered
• we run zero-downtime-push with -keep-old-app
• test-service (the new app) and test-service-venerable (the old app) are both running, but the new app is not registered in Eureka because it cannot establish a connection to Eureka due to the missing c2c policies therefore all the requests are still routed to the old service
• we run a script to set up the c2c policies
• now since the new app is able to establish a connection to Eureka it can register itself
• after some time, we run cf delete test-service-venerable to remove the old app

[mibollma]

I would find it more useful to change the flag to an enumeration e.g.
-old-app=remove|stop|none where remove is the default, none solves your case and stop solves my case #53.
What do you think?

I'm willing to help, unfortunately new to the language go.

[adamsiemion]

@mibollma Good work on the code changes! I support the idea. Actually recently we have moved away from this plugin to the tool that does the same thing but just calls cf with appropriate commands (https://github.com/adamsiemion/pcf-zerodowntimedeploy). The main reason is the fact that we use plugins for Cloud Foundry in Jenkins and Urban Code Deploy, which only support the push command and does not allow to use zero-downtime-push instead.

[mibollma]

@adamsiemion Thanks for your support. Hopefully @xoebus will find the time to take a look.