3scale proxy-config promote is not idempotent

[nmasse-itix]

current behavior

If the proxy definition has not been updated since the last promotion, the 3scale proxy promote fails with:

$ 3scale proxy-config promote nmasse-redhat api 
Error: ProxyConfig not promoted. Errors: {"version"=>["has already been taken"]}

expected behavior

$ bundle exec 3scale proxy-config promote nmasse-redhat api 
Warning: nothing to promote
$ echo $?
0

In the Ansible playbooks, I added a check to make sure there is something to promote:
https://github.com/nmasse-itix/threescale-cicd/blob/61a568068cac1063c31f24d7087c0d74a03c98f9/tasks/steps/promote.yml#L6