security/certmgr: new port

certmgr is a tool for managing certificates using CFSSL. It does the
following:

• Ensures certificates are present.
• Renews certificates before they expire.
• Triggering a service reload or restart on certificate updates.

It operates on certificate specs, which are JSON files containing the
information needed to generate a certificate.

WWW: https://github.com/cloudflare/certmgr

PR: 256992