Create targets key for the unstable repo

Create a new targets key that will be used on the unstable repo:

openssl genrsa -des3 -out private.pem 4096

The public key can be exported with:

openssl rsa -in private.pem -outform PEM -pubout -out public.pem

The private part should be stored securely.

The public part should reach micah over an authenticated channel (signed email for example). Will be nice if I'm in copy.

(from redmine: created on 2014-10-06, closed on 2014-10-06, precedes #6191 (closed))