[OBS-407]Linux agent: Handle reconfiguration

[gmann42]

What

• Handle the case when linux setup command is run again on a system on which it was previously used.
• There can be three possibilities
  • systemd service exists and is enabled: Ask user if old values should be over-written with current input
  • systemd service exists and is disabled: Ask user if old values should be over-written with current input and enable the service
  • systemd service doesn't exist: Proceed with normal flow

Ticket: https://postmanlabs.atlassian.net/browse/OBS-407

[mgritter]

Please take another pass over the errors shown to the user and make sure they clearly explain what we were trying to do, and what the user should do next (even if it's only contact us.)