`vespa prod deploy` requires named `clients.pem` file

[olaughter]

Describe the bug
The cli command for deploying a production application package contains a check for a certificate. However the check itself is just looking for a file named clients.pem. Meaning cli deploys fail when using named certs the cli fails

To Reproduce
Steps to reproduce the behavior:
In the security guide there is an example config for the services.xml file that uses the clients element to configure cert permissions. The example shows using an id and file name to have certs with names like 'serve, ingest, etc. When deploying this config to vespa cloud the error is raised:

Error: deployment to Vespa Cloud requires certificate in application package
Hint: See https://cloud.vespa.ai/en/security/guide
Hint: Pass --add-cert to use the certificate of the current application

Expected behaviour
The cli check for certs is based on the clients element, falling back to the current behaviour if not present

Screenshots
If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• OS: ubuntu
• Infrastructure: Github Actions
• Deploying to Vespa Cloud

Vespa version
CLI version used initially: 8.250.43

Additional context
Add any other context about the problem here.

[olaughter]

Hey @mpolden, I've made an attempt to resolve this, let me know what you think: #31988

[yngveaasheim]

@olaughter,
Thank you for the contribution! @mpolden is currently on PTO; I am sure he will get back to you on this when he comes back; presumably the week of August 5.

Best,
-Yngve