Make ProxyConfig promote command idempotent

[miguelsorianod]

Related to issue #173.

This PR shows a warning instead of returning an exit code different than 0 when a Staging ProxyConfig environment version has already been promoted to the Production environment.

The behavior now is:

• If ProxyConfig staging for a service does not exist (has never been updated) an error is raised
• If ProxyConfig production for a service does not exist, and promote is executed, the proxyconfiguration is promoted (it means the staging proxyconfiguration had never been promoted)
• If ProxyConfig staging and production for a service are the same a warning is shown and an exit code 0 is returned (normal exit)
• If ProxyConfig staging and production for a service are different the staging configuration is promoted to production

The output text shown in the CLI has been modified in both positive and negative outcomes to be a little more descriptive.

cc @nmasse-itix