Always leave election when stopping

[jlevesy]

What Does This PR do?

So a couple of things were wrong in how the termination was happening. This PR adresses the following issues.

• The context wasn't canceled on SIGTERM, which means prometheus-elector was never properly stopped and leaving the election
• I used Fatal, which is a mistake because it does not honor the scheduled defer introducing a risk of not leaving the election. Never use Fatal kids.
• The notifier was retyring even if the given context is canceled.
• Better logs
• Couple of DX improvements (better makefile targets, preflight checks for tooling, better requirements)
• Lowers the default lease period to 10s

This leads to much better failover time on graceful shutdowns

How to Test This PR?

make run # then kill pods and look for the logs

Good PR Checklist

• [x] Addresses one issue
• [x] Adds/Updates unit tests
• [x] Adds/Updates the documentation
• [ ] Opened against the right branch
• [x] Correctly Labeled