VMs: improve command prompts to avoid confusion between services

[dduportal]

One do not want to restart trusted.ci.jenkins.io while thinking it is ci.jenkins.io.

The prompt's hostname of both VMs is controller which is confusing and lead to human error when focusing on other production tasks.

It's caused by #3486 and #3535.

Proposals to porotect ourselves:

• Mandatory: set up prompt to have a clear hostname (FQDN or service based)
• Nice to have: set up different colors between machines (red for trusted, orange for cert.ci, blue for ci.jenkins.io)

[dduportal]

cc @daniel-beck now the controllers have the FQDN in the prompt.

I'm closing but feel free to reopen if you have more suggestions!