create updater root key and repo for unstable releases

Will need the timestamp key from kwadronaut (#6189 (closed)) and the targets key from ivan (#6190 (closed))

Generate a new root key with:

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

And initialize the repo with:

bitmask_client/pkg/tuf/init.py private.pem public.pem targets.key timestamp.key

(from redmine: created on 2014-10-06, closed on 2014-10-08, precedes #6189 (closed), precedes #6190 (closed))