Additional debugging and reliability improvements

[kisom]

• Additionally, add YAML support to cert specs.
• Add debug flag to set log level to debug.
• Add pprof endpoint on the metrics server.
• Move metrics endpoint to /metrics.
• Add cert_watching metric.
• Update web server index page.
• Use backoff in renewal.
• Die on authentication failures: these are unresolveable by certmgr and require intervention.

[lziest]

lgtm