feat: Allow replicas to be set to zero

Noticed in !49 (comment 294456431).

When I set :

kubectl scale --replicas=0 deployments/production -n minimal-ruby-app-4349298-production

and then I set PRODUCTION_REPLICAS to 0 (or REPLICAS to 0) and run a pipeline, I expect that my resultant Deployment to be deployed with zero replicas.

Manual QA

0. Before:

$ kc get deployment production -n minimal-ruby-app-4349298-production 
NAME         READY   UP-TO-DATE   AVAILABLE   AGE
production   1/1     1            1           8d

1. Test with PRODUCTION_REPLICAS 0

https://staging.gitlab.com/hello-staging/minimal-ruby-app/pipelines/12716105

Both 50% and 100% had 0 replicas as expected: ✅

kc get deployment production -n minimal-ruby-app-4349298-production 
NAME         READY   UP-TO-DATE   AVAILABLE   AGE
production   0/0     0            0           8d

2. Test with PRODUCTION_REPLICAS 1

https://staging.gitlab.com/hello-staging/minimal-ruby-app/pipelines/12716106

Both 50% and 100% had 1 replicas as expected: ✅

$ kc get deployment production -n minimal-ruby-app-4349298-production
NAME         READY   UP-TO-DATE   AVAILABLE   AGE
production   1/1     1            1           8d

3. Test with PRODUCTION_REPLICAS 2

25% had 1 replica as expected: ✅

$ kc get deployment production -n minimal-ruby-app-4349298-production
NAME         READY   UP-TO-DATE   AVAILABLE   AGE
production   1/1     1            1           8d

50% had 1 replica as expected: ✅

$ kc get deployment production -n minimal-ruby-app-4349298-production
NAME         READY   UP-TO-DATE   AVAILABLE   AGE
production   1/1     1            1           8d

100$ had 2 replicas as expected: ✅

$ kc get deployment production -n minimal-ruby-app-4349298-production
NAME         READY   UP-TO-DATE   AVAILABLE   AGE
production   2/2     2            2           8d